Error handling could be more precise

[Leamsi9]

Thanks for a package I'm enjoying sinking my teeth into.

I forgot to rerun my migrations so the web calls database was AWOL. What I got was an "Invalid Signature" error, so I naturally spent a while debugging the signature config and secret. It was not until I commented out the throw WebhookFailed::invalidSignature(); in WebhookProcessor.php that I got a missing table error, which obviously had nothing to do with the signature being invalid. Once I ran my migrations my signature was fine and all worked as intended.

Might be worth differentiating signature errors from other errors?

[freekmurze]

Feel free to PR improvements (with tests) around this 👍