http: Allow specifying a custom error handler for UploadPack

http: Allow specifying a custom error handler for UploadPack

By abstracting the error handler, this lets a user customize the error
handler for UploadPack. A customized error handler can show a custom
error message to the clients based on the exception thrown from the
hook, create a monitoring system for server errors, or do custom
logging.

Change-Id: Idd3b87d6bd471fef807c0cf1183e904b2886157e
Signed-off-by: Masaya Suzuki <masayasuzuki@google.com>