pullrequests: detect invalid reviewers and raise HTTPBadRequest

Normally, the creation of a pullrequest with invalid reviewers is not
possible because the list of reviewers is populated from a form element
that only shows valid reviewers.
However, if creating a pullrequest through an API call, invalid reviewers
can be specified but would not be detected. The reviewer would be encoded
in the database as 'NULL'/None, and opening such a pull request would cause
a server error.

Instead, detect invalid reviewers at pullrequest creation/update time and
raise HTTPBadRequest.