Provide a more detailed error message when an SSLException occurs on checkout

When you checkout a repository with a ssl secured and there is a problem with the truststore etc. the displayed error message is not enough to determine that there is a problem with the certificates:

Error message now:
Failed to connect to https://domain.com/git/repository.git using credentials Gitblit user account

Error message after change (e.g. when there is no certificate in truststore):
sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target