Handling unhandled exceptions

If a handler raises an exception during its lifetime, this provides a
way in the application instance to log the error and return an error
message to the client instead of the generic Google App Engine error
screen.
Also use 'failed' statistics for unroutable paths

Ceases the use of the 'unresolved' statistics category and merges it
into with the 'failed' statistics.
Disable on-screen debugging

This switches the default for on-screen debugging to be disabled. Error
messages can always be seen on the command line or by temporarily
toggling this back on.