LfsProtocolServlet: Allow getLargeFileRepository to raise exceptions

According to the specification [1] the server may return the following
HTTP error responses:

- 403: The user has read, but not write access.
- 404: The repository does not exist for the user.
- 422: Validation error with one or more of the objects in the request.

In the current implementation, however, getLargeFileRepository can only
return null to indicate an error. This results in the error code:

- 503: Service Unavailable

being returned to the client regardless of what the actual reason was.

Add exception classes to cover these cases, derived from a common base
exception, and change the specification of getLargeFileRepository to throw
the base exception.

In LfsProtocolServlet#post, handle the new exceptions and send back the
appropriate HTTP responses as mentioned above.

The specification also mentions several other optional response codes (406,
429, 501, and 509) but these are not implemented in this commit. It should
be trivial to implement them in follow-up commits.

[1] https://github.com/github/git-lfs/blob/master/docs/api/v1/http-v1-batch.md#response-errors

Change-Id: I91be6165bcaf856d0cefc533882330962e2fc9b2
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>

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