Improve error messages for repository creation RPCs

The RPC functions to create repositories from bundles and URLs have loads of ways to fail, but they don't provide any nice error messages in case they do because of an error in the spawned Git command. As users will only see a generic message like "CreateRepositoryFromURL: clone cmd wait: exit status 128", finding the root cause is next to impossible.

Let's improve the situation by including stderr from the respective Git commands in the error message. This should hopefullly give useful hints as to why a given operation fails, instead of lumping everything together under a single exit code.

References #1403 (closed)