Provide more verbose output when iWaitForAjaxToFinish() fails.

[marcoscano]

Currently error messages when an AJAX call fails are quite short:

AfterStep # Drupal\DrupalExtension\Context\MinkContext::afterJavascriptStep()
      Unable to complete AJAX request. (RuntimeException)

or

And I wait for AJAX to finish                                                                                 # MinkContext::iWaitForAjaxToFinish()
      Unable to complete AJAX request. (RuntimeException)

Could we add to that exception message more information about the event that failed?

[brooke-heaton]

iWaitForAjaxToFinish appears to be inherently problematic function. All tests are now failing at this step for me on 4.1 using selenium/standalone-chrome:3.141.59-oxygen

[mxr576]

Related #557