Better error messages when fields missing on user model

[benjaminwootton-zz]

I have implemented authlogic 2 or 3 times but always hit a sticky patch which turns out to be due a field missing on the model:

undefined method login?' - You missed your crypted password field on user model undefined methodlogged_out?' - You potentially are trying to use logout_on_timeout option which depends on last_request_at

I had to dig through source code to work out the second of those.

Explcit check for these messages and a suggestion message would really help on this.