Enable H302 check

Enable H302 (import only modules) which passes without any changes to files.

Change-Id: Ibcc609ffad404e8d73a4828b7b18892444e7454d
Clean up DatabaseMigrationError

Exception DatabaseMigrationError from glance/common/exception.py
is unused, so we get rid of it.

Change-Id: I36f70dcc7d5b282d3c9d9e82a0c3f673d008e71e
Merge "Enable H302 check"
Merge "Clean up DatabaseMigrationError"