Don't display text if we can't help

[davidwagner]

We should only display our custom text if we have specific advice that should help. Currently it always displays a message, even if it doesn't match any of the conditions in the config file. That leads to something like: "[X's] are usually because of: [blank]". I think we should avoid that, and if the exception doesn't match anything in the config file, don't show our custom text -- just show the standard exception text that would have been displayed in the absence of d8error.