Currently migrate.versioning.api.upgrade() raises KeyError
instead of sqlalchemy-migrate specific exception if migration
script file is not present in migration repository.
Raised migrate.exception.VersionNotFoundError if the specified
migration script does not exist in the repository. Made
VersionNotFoundError exception class as a subclass of KeyError
in order to avoid breaking existing users looking for KeyError.
Related-Bug: #1546441
Change-Id: I0210d56a6e85f03c44cea027f50863faaf050c1d
2a32681036