Provide example for failing the webhook

[tilsammans]

There is no documented way to fail a webhook. Reading the code I found that exceptions are appropriate here, so added a small example to the README to make that explicit.

[wingrunr21]

Hi,

Thanks for the contribution. I think better documentation around the approach would be that Griddler is itself simply a controller action. Therefore, any side effects that cause an error code other than a 200 in other Rails contexts would also apply to Griddler. What do you think?

[tilsammans]

Sure.