Interactive Auth: Return 401 from for incorrect password

This requires a bit of fettling, because I want to return a helpful error
message too but we don't want to distinguish between unknown user and invalid
password. To avoid hardcoding the error message into 15 places in the code,
I've had to refactor a few methods to return None instead of throwing.

Fixes https://matrix.org/jira/browse/SYN-744