Fix #941 -- allow errorhandlers for HTTPException

Originally i intended to rewrite the error handling system to register an
error handler for each subclass of HTTPException on initialization of the
Flask object, removing the distinction between HTTP exceptions and other
exception types. However, with the current behavior of preferring the
first-registered handler when an error occurs, doing this without compromises
would require changing this to preferring the last-registered handler, a
backwards-incompatible change.

The following is a very minimal fix to only make Flask prioritize the user's
error handlers over others. I am not sure which performance implications this
has. Maybe a major rewrite of the error handling system is still desirable
(with more complex prioritization of errorhandlers -- something based on
"distance" in inheritance tree?), but at the moment i don't see a reason to do
so, given that the current system is primitive enough to be understandable,
while still usable.