qmlls: clean up warnings/debug output

The output from qmlls is currently very chaotic and confusing. I suspect most of it is debug output leftovers that can be demoted to warnings or even debugging messages.
For example:
"Cannot proceed: current QML document is invalid! Fix all the errors in your QML code and try again."
that is shown as an error but actually should be a debugging message, as it only is about qmlls waiting for the document to be valid before being able to work on it. This usually happens when users are typing code, as then there are some moments where the QML code is incomplete and therefore invalid.