Differentiate input errors from other internal errors

[RussellLuo]

There are two types of errors:

1. Input errors, which should be treated as soft failures
   • Not found: the input username does not exist
   • Invalid Credentials: the input password does not match the given username
2. Internal Errors, which should be treated as hard failures
   • The given ROUser is not valid
   • The given BaseDN does not exist
   • The given Host can not be connected
   • The given Filter has a syntax error
   • Other errors from the LDAP server

Also, the existing input errors can not be compared in an idiomatic way.