Add more error codes, messages and error checking.

I understand we need the cast on the error returned by CreateSeries
(retErr) because it's of type error and we want access to the
ManagerError bits, but that should not be the case for testErr, which is
in the test struct. AIUI, we could change the struct in the test to use
waddrmgr.ManagerError instead of error and then this code would look
like this:

if err.(waddrmgr.ManagerError).ErrorCode != test.err.ErrorCode {
    ...
}