CSS file paths for default errors should be relocatable.

With Dancer2 apps being able 'relocatable' (able to be mounted via URLMap 
without adding a prefix), ensure the default error content also
adjusts paths to their css files appropriately.

Includes a simple test case. Also update dispatcher.t and dancer-test.t
so there is sufficient content in their request envs to calculate uri_base
to shhh use of uninitialized var warnings.