Better error message, avoiding UnicodeDecodeError (cctbx#763)

Catch UnicodeDecodeError and raise more appropriate InvalidExperimentListError

Fixes dials/dials#2453

newsfragments/763.bugfix

@@ -0,0 +1 @@
+Raise a more suitable error message when failing to load an experiment list.

src/dxtbx/model/experiment_list.py

@@ -870,9 +870,14 @@ def from_json_file(filename, check_format=True):
         """Load an experiment list from a json file."""
         filename = os.path.abspath(filename)
         directory = os.path.dirname(filename)
-        with open(filename) as infile:
-            return ExperimentListFactory.from_json(
-                infile.read(), check_format=check_format, directory=directory
+        try:
+            with open(filename) as infile:
+                return ExperimentListFactory.from_json(
+                    infile.read(), check_format=check_format, directory=directory
+                )
+        except UnicodeDecodeError:
+            raise InvalidExperimentListError(
+                f"Cannot interpret {filename} as an ExperimentList"
             )
 
     @staticmethod