Expand BrowserTestBase with error handling support

Yeah spent some time on this on Friday on a client test, will ping you for a chat with some ideas

+ if ($driver)
 return $session;

You have an extra if that I think is preventing the session return

Just for reference we solved this a bit differently in Rules for now: https://github.com/fago/rules/blob/8.x-3.x/tests/src/Functional/RulesBro...

We have created methods on our browser test base class that perform web requests. At the end of each method we check the response for assertion headers. Primitive, but works.

fago also complained about this and we are seeking other solutions in #2659808: Improve browser tests to check for PHP errors and/or errors in watchdog.

1. +++ b/core/lib/Drupal/Core/Test/HttpClientMiddleware/TestHttpClientMiddleware.php
   @@ -31,7 +33,21 @@ public function __invoke() {
   +          ->then(function (ResponseInterface $response) use ($request, $options) {
   

   $options doesn't look used?

2. +++ b/core/lib/Drupal/Core/Test/HttpClientMiddleware/TestHttpClientMiddleware.php
   @@ -31,7 +33,21 @@ public function __invoke() {
   +                  $parameters = unserialize(urldecode($header_value));
   

   If simpletest module is enabled on a production site (yep someone might do that) - this could lead to unserializing user provided data. I think we should be checking drupal_valid_test_ua() here too - we're already passing in $request (but not using it?) - that way this code can't be run outside of the test-runner?

3. +++ b/core/lib/Drupal/Core/Test/HttpClientMiddleware/TestHttpClientMiddleware.php
   @@ -31,7 +33,21 @@ public function __invoke() {
   +                  throw new \Exception($parameters[1] . ': ' . $parameters[0] . "\n" . Error::formatBacktrace([$parameters[2]]));
   

   Should we be defensive here and make sure that count($parameters) === 3.
   Personally I'd prefer list($something, $something_else, $something_else_again) = $parameters over the magic 0,1,2.

Thanks mate, looks good to me

Committed fde19db and pushed to 8.0.x, 8.1.x, 8.2.x since this only affects test code (and pretty experimental test code at that). Thanks!