Improve error message when bundle server doesn't serve a bundle

[anderseknert]

When pointing OPA to fetch bundles from an endpoint that doesn't serve bundles (likely due to typo or misconfiguration in the remote), the error message looks like this:

{"level":"error","msg":"Bundle load failed: bundle read failed: archive read failed: gzip: invalid header","name":"authz","plugin":"bundle","time":"2022-01-25T06:28:49Z"}

This has led many users to believe there is something wrong with the bundle packaging itself, leading them to debug the bundle creation process, when the real problem is that the remote doesn't serve the bundle in the first place.

It would be good if we printed this error message only when a broken bundle was served. If we can't differentiate, we might want to print just something like "no bundle found at remote or invalid bundle format".

[stale]

This issue has been automatically marked as inactive because it has not had any activity in the last 30 days.

[anderseknert]

I think we can improve the error message by looking at the Content-Type response header. If it isn't application/octet-stream (as a tarball is just binary data) but something like text/html, we can print a more informative message.