Always return the errors and warnings from erlydtl

This allows us to use `rebar_base_compiler:ok_tuple/3` and
`rebar_base_compiler:error_tuple/5` when returning from
`rebar_erlydtl_compiler`.

This is the same as #36 except replacing `report` with `return` at src/rebar_erlydtl_compiler:172 as suggested in the code review.