PasswordCallback should give more errors than "permision denied"

[umialpha]

In our ssh server, we integrate third-party authn system. When error occurs, it always prompts "permision denied", which confuses our users, meanwhile, we have to inspect server logs to give our users an explaination.
So, we need to prompt the details errors to our users, not just "permision denied".

I think we can fix this by adding a callback like "PasswordCallbackWithError" to make it backward compatible.
If this thought sounds ok, i can help to this feature.

[gustavosbarreto]

backward compatible? please do it

[umialpha]

@gustavosbarreto hi, I created a mr, plz take a look :)
#187