add PasswordAuthE func, which is like PasswordAuth but returns error instead of bool

[umialpha]

Add PasswordAuthE function, because sometimes ssh server should returns detailed error than "permission denied". if PasswordAuthEand PasswordAuth both set, PasswordAuthE will overwrite PasswordAuth

[gustavosbarreto]

@umialpha What do you think about this callback being generic for all of authentication errors (password, publickey...)?

[umialpha]

@umialpha What do you think about this callback being generic for all of authentication errors (password, publickey...)?

ok, let me rethink how to unify them