error on non-existent Git repository is very non-specific

[dlenski]

When I try to open a Repository using a path that doesn't actually contain a Git repo, the error is a very non-specific KeyError:

>>> print(pygit2.__version__)
0.24.1
>>> os.mkdir('/tmp/foobarbaz')
>>> gitrepo=pygit2.Repository('/tmp/foobarbaz')
Traceback (most recent call last):

  File "<ipython-input-30-782283f588bb>", line 1, in <module>
    gitrepo=pygit2.Repository('/tmp/foobarbaz')

  File "/usr/local/lib/python2.7/dist-packages/pygit2/repository.py", line 64, in __init__
    super(Repository, self).__init__(path, *args, **kwargs)

KeyError: '/tmp/foobarbaz'

It would be helpful to replace this with a more specific GitError, so that the error from Git can be distinguished from a "normal" KeyError in a statement like this:

>>> repos = { "repo1":"/path/to/repo1", "repo2":"/path/to/repo2" }
>>> gitrepo = pygit2.Repository( repos["repo3"] )