Prevent QOpenGLShaderProgram::link() from showing unnecessary warnings.

If the liking didn't fail there's no need to print out warnings. The
warnings can still be accessed by the application after calling link()
through the log() function.

This prevents warnings such as these from appearing:

QOpenGLShader::link: "No errors."

Task-number: QTBUG-31326
Change-Id: I03c9be5dfada8822c3ab1c3610eac2fc0a91410b
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>