QMediaNetworkPlaylistProvider: Upon error parsing, stop parsing.

When an error is found parsing a playlist, stop parsing. This will also prevent
the emission of the "loaded" signal when the parser finishes.

Some of the newly added testcases do not yet pass because the parser is overly
strict. These improvements are being tracked in QTBUG-40515.

Change-Id: I5c96b7eb488996f28eebd7b6c643940de8e2e0b9
Done-by: Andres Gomez <agomez@igalia.com>
Task-number: QTBUG-40513
Reviewed-by: Andrew den Exter <andrew.den.exter@qinetic.com.au>