Raise exception if pdns_server and/or auth_token are not provided (#755)
Raise exception if pdns_server and/or auth_token are not provided (#755)

* Raise exception if pdns_server and/or auth_token are not provided

Currently if `pdns_server` is not defined we are going to fail with unclear exception (`None` has no `ends_with`). This is to add generic provider exceptin type and check for the presence of required attributes.

* Fix whitespaces
AnalogJ/lexicon,AnalogJ/lexicon