Default onPageNotFound priority should be -1 or less.

[Flamenco]

Default onPageNotFound priority should be -1 or less.

This will allow other plugins to set the priority to 0 on onPageNotFound and still be called when they are loaded after the default fallback of this plugin.

[rhukster]

if you can provide me a PR, I can test it. cheers.

[Flamenco]

#31