Does anyone have an idea what may have caused this, and if so would it be easy to put the error messages back in?
When did it break?
I did some work right after 2.10 to support custom logging, and one thing I started doing, but did not finish, was routing all OSG and related Simgear-log output to its own place (in addition to the normal ones) so that it could be show in a 'render failures' log. (And hence hopefully *easier* to spot OpenGL/shader related problems).
As far as I recall, that work is incomplete (I didn't have time to build the actual dialog/extend the rendering dialog) but it should also be disabled. Maybe I committed it enabled by accident? Have a look in fg_os_osgviewer.cxx where OSG is initialised.
If this is the cause, it's been that way since January, and I assume you would have compiled from source since then.