Prevent 500 if hook url is down when testing a hook, fixes #7376

##### What does this MR do?

This PR catches errors if the target webhook url is not down. If so, an error message is displayed at the bottom of the page.

##### Why was this MR needed?

Currently, if the user adds a webhook URL which is down, a 500 error is raised when pressing the `Test Hook` button.

##### What are the relevant issue numbers / Feature requests?

This PR fixes #7376