transport: Add ReceiveCommandErrorHandler

transport: Add ReceiveCommandErrorHandler

This gives a chance to handle an exception for a user. For example, when
an IOException is thrown while executing
`walk.parseAny(cmd.getNewId())`, it's always handled as
REJECTED_MISSING_OBJECT. However, IOException can mean a Git storage IO
error. By introducing an error handler class, a user can add a custom
error handler for these cases.

Change-Id: I3e03a536e1d8e137cb0f6e596d71642e72adde9e
Signed-off-by: Masaya Suzuki <masayasuzuki@google.com>