[#93] Added handling for unhandled and unexpected abstract forms.

The error message reported on #93 was misleading since the actual error was an unhandled `ann_type` in an [`escalus / include / escalus.hrl`](https://github.com/esl/escalus/blob/af0a385b53da5966fb2d031d8f9e5f121c91af5a/include/escalus.hrl#L28).