The KHR_no_error extension is about avoiding OpenGL error handling for GL games/applications known to be error-free. By avoiding the OpenGL error code, the CPU usage may be reduced and there is the potential for power-savings too. In place of the OpenGL error handling though is the possibility of undefined behavior, if the application is not in fact error-free.
The functionality with Mesa Git can be enabled via the MESA_NO_ERROR=1 environment variable, but it will be interesting to see just how many Linux OpenGL games are "error free" where they can be safely enabled. (Yes, many of you out there probably believe error-free OpenGL is an oxymoron.)
More details on this KHR_no_error extension are available from the OpenGL registry.