Be more informative in case of Meson errors
I was trying to get (re)started on a project that I didn't touch for some time but Builder was refusing to build the project with just one thing popping up on the "Information panel": "Invalid JSON received from meson introspect"
After asking on IRC about it, I was able to see the true error behind the problem:
mesonbuild.mesonlib.MesonException: Build directory has been generated with Meson version 0.44.0, which is incompatible with current version 0.48.1. Please delete this build directory AND create a new one.
Either Builder should detect this error and "upgrade" the builddir, or should this info to the user so he can fix the error manually. (indicating where the actual builddir is)