Fix repo discovery for private/non-existing repos

[hagenw]

Closes #333

Make sure that audb does not raise a backend error when looking for databases on different backends, including ones where no permissions are given or that do not exist. For the later you might argue that it is ok to raise an error, but on Artifactory we cannot differentiate between having no access rights and non-existing repos. As we usually share the same config file for all users, it seems more important to me to not fail in the case of missing access rights.

The pull request first adds a test that fails for the current main branch, and changes the code afterwards to no longer raise an error.

[codecov]

Codecov Report

Merging #334 (6ae47ae) into main (ba8c0d9) will not change coverage.
The diff coverage is 100.0%.

Additional details and impacted files

Files	Coverage Δ
audb/core/utils.py	100.0% <100.0%> (ø)

[hagenw]

@ChristianGeng as this issue is quite urgent and affects most users, I will skip the review for it in order to make a new release as soon as possible.