Modify release script to distinguish git tag errors

[LeiGlobus]

When I released 2.32.0 earlier this week, the git tag command in release.sh failed(my gpg key, only used for tags, expired a few weeks earlier), but I missed the error message amongst the release output:

...
error: gpg failed to sign the data
error: unable to sign the tag
...

The script thinks it's a 'tag already exists' error and asks to confirm, which I answered 'y' without becoming suspicious of why the tag would already be pre-existing. In the end, the packages were released but the tag wasn't pushed.

Instead, the user should be prompted for 'already exists' situations, but the script should abort for other, unknown issues.

[khk-globus]

Good call!

Please double check the arguments to Git in the subshell, but in concept, this is a nice addition. Thanks!

[khk-globus]

FWIW, inside of the subshell, there's not a need to escape the quotes. Consider:

TAG_STDERR="$(git tag -s -m "v$VERSION" "$VERSION" 2>&1 > /dev/null)"

Please double check me, but I think the escapes might even include the quotes in what's given to Git:

$ show_args a \"b\" c
ARG 0: show_args
ARG 1: a
ARG 2: "b"
ARG 3: c

$ SOME_VAR="$(show_args a \"b\" c)"

$ echo -e "$SOME_VAR"
ARG 0: show_args
ARG 1: a
ARG 2: "b"
ARG 3: c

[khk-globus]

Mildly odd to make that 4 arguments to echo instead of 1, but "meh."