ft: create error handler method

[kennedymwavu]

Add a method to set the error handler

Closes #63

[JohnCoene]

Do you mind set_error instead of set_error_handler that would make it more consistent with other methods (e.g.: set_404 ~set_404_handler~).

Also maybe check that the handler is indeed a function (there is an {assertthat} function for that somewhere in the package.

[kennedymwavu]

Very sensible. Making the suggested changes.

[kennedymwavu]

Don't merge yet... Let me make one more change: Use all() instead of && for backward compatibility.

[kennedymwavu]

LGTM now.

[JohnCoene]

Thanks!