Fail earlier if archive download failed

Simplify debugging this traceback (which was just an intermittent
network error):

https://github.com/galaxyproject/galaxy/actions/runs/12374234876/job/34536295999

```
______________________ ERROR at setup of test_build_index ______________________

cls = <class 'tarfile.TarFile'>, name = None, mode = 'r'
fileobj = <gzip on 0x7f349cee7d00>, compresslevel = 9, kwargs = {}
GzipFile = <class 'gzip.GzipFile'>

    @classmethod
    def gzopen(cls, name, mode="r", fileobj=None, compresslevel=9, **kwargs):
        """Open gzip compressed tar archive name for reading or writing.
           Appending is not allowed.
        """
        if mode not in ("r", "w", "x"):
            raise ValueError("mode must be 'r', 'w' or 'x'")

        try:
            from gzip import GzipFile
        except ImportError:
            raise CompressionError("gzip module is not available")

        try:
            fileobj = GzipFile(name, mode + "b", compresslevel, fileobj)
        except OSError:
            if fileobj is not None and mode == 'r':
                raise ReadError("not a gzip file")
            raise

        try:
>           t = cls.taropen(name, mode, fileobj, **kwargs)

/opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/tarfile.py:1854:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
/opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/tarfile.py:1831: in taropen
    return cls(name, mode, fileobj, **kwargs)
/opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/tarfile.py:1694: in __init__
    self.firstmember = self.next()
/opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/tarfile.py:2578: in next
    tarinfo = self.tarinfo.fromtarfile(self)
/opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/tarfile.py:1282: in fromtarfile
    buf = tarfile.fileobj.read(BLOCKSIZE)
/opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/gzip.py:292: in read
    return self._buffer.read(size)
/opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/_compression.py:68: in readinto
    data = self.read(len(byte_view))
/opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/gzip.py:479: in read
    if not self._read_gzip_header():
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <gzip._GzipReader object at 0x7f349cee79a0>

    def _read_gzip_header(self):
        magic = self._fp.read(2)
        if magic == b'':
            return False

        if magic != b'\037\213':
>           raise BadGzipFile('Not a gzipped file (%r)' % magic)
E           gzip.BadGzipFile: Not a gzipped file (b'<!')

/opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/gzip.py:427: BadGzipFile

During handling of the above exception, another exception occurred:

    @pytest.fixture(scope="module")
    def community_file_dir():
        extracted_archive_dir = tempfile.mkdtemp()
        b = BytesIO(requests.get(URL).content)
>       tarfile.open(fileobj=b, mode="r:gz").extractall(extracted_archive_dir)

test/unit/tool_shed/test_shed_index.py:30:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
/opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/tarfile.py:1801: in open
    return func(name, filemode, fileobj, **kwargs)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

cls = <class 'tarfile.TarFile'>, name = None, mode = 'r'
fileobj = <gzip on 0x7f349cee7d00>, compresslevel = 9, kwargs = {}
GzipFile = <class 'gzip.GzipFile'>

    @classmethod
    def gzopen(cls, name, mode="r", fileobj=None, compresslevel=9, **kwargs):
        """Open gzip compressed tar archive name for reading or writing.
           Appending is not allowed.
        """
        if mode not in ("r", "w", "x"):
            raise ValueError("mode must be 'r', 'w' or 'x'")

        try:
            from gzip import GzipFile
        except ImportError:
            raise CompressionError("gzip module is not available")

        try:
            fileobj = GzipFile(name, mode + "b", compresslevel, fileobj)
        except OSError:
            if fileobj is not None and mode == 'r':
                raise ReadError("not a gzip file")
            raise

        try:
            t = cls.taropen(name, mode, fileobj, **kwargs)
        except OSError:
            fileobj.close()
            if mode == 'r':
>               raise ReadError("not a gzip file")
E               tarfile.ReadError: not a gzip file

/opt/hostedtoolcache/Python/3.8.18/x64/lib/python3.8/tarfile.py:1858: ReadError
------------------------------ Captured log setup ------------------------------
DEBUG    urllib3.connectionpool:connectionpool.py:1022 Starting new HTTPS connection (1): github.com:443
DEBUG    urllib3.connectionpool:connectionpool.py:475 https://github.com:443 "GET /mvdbeek/toolshed-test-data/blob/master/toolshed_community_files.tgz?raw=true HTTP/1.1" 503 54894
```

test/unit/tool_shed/test_shed_index.py

@@ -26,7 +26,9 @@ def whoosh_index_dir():
 @pytest.fixture(scope="module")
 def community_file_dir():
     extracted_archive_dir = tempfile.mkdtemp()
-    b = BytesIO(requests.get(URL).content)
+    response = requests.get(URL)
+    response.raise_for_status()
+    b = BytesIO(response.content)
     tarfile.open(fileobj=b, mode="r:gz").extractall(extracted_archive_dir)
     try:
         yield extracted_archive_dir