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/genericprojectmanager/genericproject.cpp

@@ -565,7 +565,7 @@ GenericProjectFile::GenericProjectFile(GenericProject *parent, QString fileName)
 GenericProjectFile::~GenericProjectFile()
 { }
 
-bool GenericProjectFile::save(const QString &)
+bool GenericProjectFile::save(QString *, const QString &)
 {
     return false;
 }