Disallow lost exceptions (W0150).

* Exceptions should be explicitly caught and handled.
Handle exceptions explicitly.
Merge pull request #718 from Renelvon/no_lost_exc

Disallow early return from finally blocks (W0150)
Flag implicit relative imports (W0403).
State relative imports explicitly.

* Reorder imports in accordance to PEP8.
* Replace calls to privtopub with calls to
  privkey_to_pubkey for consistency.