Refactored Exception Handling, mostly tested authorization request handler

* Internal error conditions now raise IndieAuthException
* Bubbled unknown exceptions converted to generic IndieAuthException
* Exceptions passed to overridable handler, turned into response
* Wrote many more tests, fixed a variety of problems

tests/templates/code_exception_response.txt.php

@@ -0,0 +1,4 @@
+<?php
+/** @var Taproot\IndieAuth\IndieAuthExcepton $exception */
+/** @var Psr\Http\Message\ServerRequestInterface $request */
+?><?= $exception->getCode() ?>