client.py: replace a never-used GitError with a ClientError.

Nobody ever tried calling that function, so it's really just an assertion
that never triggered.  Which is good, because it was trying to throw an
exception that wasn't available in the current namespace.

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>

lib/bup/client.py

@@ -309,7 +309,7 @@ def close(self):
         return id
 
     def abort(self):
-        raise GitError("don't know how to abort remote pack writing")
+        raise ClientError("don't know how to abort remote pack writing")
 
     def _raw_write(self, datalist, sha):
         assert(self.file)