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.

Task-number: QTBUG-40513
Change-Id: Ia814864d0d546806219993f0b727761d5d4e7903
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>

tests/auto/unit/qmediaplaylist/testdata/test.pls

@@ -0,0 +1,10 @@
+[playlist]
+
+File1=http://test.host/path
+Title1=First
+Length1=-1
+File2= http://test.host/path
+Title2=Second
+Length2=-1
+
+NumberOfEntries=2