Fix typo in dsl halt subtest name
Wrap 'pass' keyword in Return::MultiLevel logic so it returns when executed.

The pass keyword is documented as "returning immediately" (like forward and halt).
Move the handling of 'pass' to the context object to wrap the Return::MultiLevel
logic around it to effect that return.

Adds a basic test.
Ensure send_error returns from route when executed (as documented).

Resolves #570.
Ensure send_file returns from route handler immediately as documented.

Also cleaned up the use of send_file within apps with prefixes (prefixes
get removed when looking for the file on disk).
Cleanup DSL; s/self->app->context/self->context/
Reflect changes
Merge branch 'pr/send_error_return'