add errorString output argument to IFile::save()

this centralizes error reporting (in fact, in most cases it adds any in
the first place).

Task-number: QTCREATORBUG-1619

src/plugins/qmlprojectmanager/qmlprojectfile.cpp

@@ -46,7 +46,7 @@ QmlProjectFile::QmlProjectFile(QmlProject *parent, QString fileName)
 QmlProjectFile::~QmlProjectFile()
 { }
 
-bool QmlProjectFile::save(const QString &)
+bool QmlProjectFile::save(QString *, const QString &)
 {
     return false;
 }