Add Error impl, helpers to LdapResult
Add Error impl, helpers to LdapResult

* LdapResult has everything to make it an Error struct, just
  enable it.

* Add helper methods which turn an LdapResult into a Result of
  itself according to the value of rc.

* For now, LdapResult as an error will be wrapped in io::Error
  (as all errors currently are).

inejge/ldap3,inejge/ldap3,inejge/rust-ldap