Allow override of "access denied" message

Created hook_webform_build_access_denied_alter().

Thanks. I've made a few tweaks to the documentation.

Thank you. I will do a full test on Monday.

The patch works. I was able to change the output using hook_preprocess_HOOK(). One coding standards issue:

// This:
'variables' => ['message' => '', 'attributes' => [], ],

// Should be:
'variables' => ['message' => '', 'attributes' => []],

Or the array could be broken-up with one element per line.

Tests that fail in #10 are failing on the branch as well.

Would you like me to fix the coding standards and put this in the merge request?

Sorry, the last one was missing the new template.

I'm trying to use this in our real-world application now. Inside hook_preprocess_webform_access_denied(), I can change the "access denied" message. However, for my use case, I need access to the Webform object. Is there any way to get this or would buildAccessDenied() need to include $webform as a property in $build?

The attached patch does what we need. Is there another way that a hook_preprocess_webform_access_denied() implementation could get the current Webform object?