Add support for auth-backend UserQueryErrors.
Extensions with custom AuthBackends can now raise a UserQueryError,

with a detailed message, from within a query_users() method.  It will

be caught and turned into a web API USER_QUERRY_ERROR.

All unit tests pass except for some VCS-specific ones, which are skipped.
Manually verified that a UserQueryError with a custom message passed

from an extension gets included in an AJAX error response.