Improve ODL ML2 Exception Handling

Raise a MechanismDriverError when authentication with ODL fails.
Re-raise HTTPError in "sync_resources" for exceptions whose status code is not 404.

Closes-Bug: #1311758
Change-Id: I99d2c4f270007c369024525ba4e6188d232a0b13
(cherry picked from commit 7db44edd3c5ad4aea508c3cc74a8b0b8d785c26b)
Merge "Improve ODL ML2 Exception Handling" into stable/icehouse