Improved error handling

[midahp]

• AppFinder will now return a 404 Not Found response instead of causing an exception if an app could not be found. A message will be logged with log level INFO
• Exceptions handled by Horde_ErrorHandler will now return a reponse with status code 500 Internal Server Error. Previously it was 200
• Refactored html creation logic in Horde_ErrorHandler
• Implemented ErrorFilter middleware and added to top of middleware stack. Exceptions caused while using the middleware stack will now be handled by this class instead of Horde_ErrorHandler.
• ErrorFilter middleware will return a json encoded response if the requests Accept header contains application/json. Otherwise the usual html error response will be returned.

[midahp]

@ralflang This is the PR I was talking about earlier

[ralflang]

In principle I am for it.

One objection: I would have put the BC break into /src and left the original /lib semantically unchanged but horde/core is a mess of glue stuff and at least it's changed consistently. I hope we don't break traditional error pages outside controller framework. Let's merge it.