CSS file paths for default errors are apended to request.uri_base

With Dancer2 apps being able to be mounted via URLMap without adding
a prefix into the application, 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.
Update Changes
Merge branch 'veryrusty-Pr/relocatable_default_errors'