Make LDAP bind failure cause an ldap_error, rather than an invalid_credentials

Fixes #51
`Net::LDAP#search` returns nil if the operation was not successful: https://github.com/ruby-ldap/ruby-net-ldap/blob/master/lib/net/ldap.rb#L722-765

Previously, this was just not entering the `if` and returning the default result of `false`.  If search can successfully connect, but cannot find results (bad username/password in the params), `#search` returns an empty array, and we still get `invalid_credentials` as a result.