core: log message subject as well

When dealing with complaints as to reasons why an account has been
disabled it will be useful to be able to say "You were sending emails
with the subject Free Viagra so it is safe to assume your account was
taken over by a spammer".
core: include the email address in the error message

When looking at the logs a message of "Not a valid user" doesn't
actually help debugging much in case it was a valid user. Include a
repr'd version of the address to make it more obvious what has been
going on.
Merge branch 'master' of github.com:damoxc/vmail