MDL-66979 behat: Handle driver stop failure properly

An case was found where the webdriver stop() call could fail in an
AfterScenario hook, leading to a complete rerun if no other errors were
experienced.

lib/tests/behat/behat_hooks.php

@@ -660,7 +660,24 @@ EOF;
      * @AfterScenario
      */
     public function reset_webdriver_between_scenarios(AfterScenarioScope $scope) {
-        $this->getSession()->stop();
+        try {
+            $this->getSession()->stop();
+        } catch (Exception $e) {
+            $error = <<<EOF
+
+Error while stopping WebDriver: %s (%d) '%s'
+Attempting to continue with test run. Stacktrace follows:
+
+%s
+EOF;
+            error_log(sprintf(
+                $error,
+                get_class($e),
+                $e->getCode(),
+                $e->getMessage(),
+                format_backtrace($e->getTrace(), true)
+            ));
+        }
     }
 
     /**