Fix error results of event callbacks

[mkj]

Currently it's assumed an application will exit if an event callback returns an error (for example .hostkeys()? if buffer space isn't available).

It might be worth storing that error somewhere and also making the main progress() loop propagate it out as well. Then the callbacks would only return errors if there was a problem with the data provided by the API.

Need to decide whether non-utf8 errors etc from password() or username() are fatal.