Please add more error types

[Hain2000]

I wish there were more kinds of error types in pygit2, so that I could do more things with them https://github.com/libgit2/libgit2/blob/198a1b209a929389c739a8a6abef13e717fdfda9/include/git2/errors.h#L56 https://github.com/libgit2/pygit2/blob/bf2d7542846fc444b86d02d44536103b27b9141f/pygit2/errors.py