Treat warnings as errors in qmltc

qmltc now fails if any error or warning is encountered. Ideally, it should figure which warnings are critical (making them errors) and then leave all others as warnings and ignore them. Baseline is to just do a "-Werror -Wall" analogy right now.
So: qmllint's warnings are qmltc's errors.
Depending on how much of a change it is, we might be able to pick it to 6.3 (and we really should try doing this).