Since [1], the git-lfs specification allows the server to return
HTTP 507 if there is insufficient storage for the uploaded object(s).
Add a new exception class, which implementations may throw from the
getRepository() method, causing HTTP 507 to be returned to the client.
[1] https://github.com/github/git-lfs/pull/1473
Change-Id: If5bc0a35fcf870d4216af6ca2f7c8924689ef9c5
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>