QGLWidget/QGLContext::makeCurrent() should return a bool

A call to wglMakeCurrent() made by QGLWidget which failed is ignored silently and no status or indication is returned to the program. This is a severe bug since the program doesn't have any way to know something went wrong. The docs mentions "In some very rare cases the underlying call may fail. If this occurs an error message is output to stderr." This is not sufficient. It should do something more sensible like making isValid() return false. This "rare" case happens when for example running an app with Compuware DevPartner Performance Analysis.