validators: don't catch all Exceptions as invalid clone URIs, be specific

When adding a new repository with a remote clone URI, the URI will be
validated in some way. Several exceptions could occur during that
validation.

Previously, the code would catch based on 'Exception', which means that
_any_ exception would cause the URI to be found invalid. This means that
errors in the code (e.g. related to Python 3 conversion) were also
categorized as 'invalid clone URI'. And thus, the tests that test an
actually invalid URI would pass, even though there was a bug.

Now, things have been refactored so it only is relevant to catch
InvalidCloneUriException. Any other exception will now yield a 500 Internal
Server Error, as expected.