The need for SERENITY_ROOT was basically eliminated in
73c953b674. The existing guess
'git rev-parse --show-toplevel' should be correct in all conceivable cases.
Most code just assumes the layout in git, or depends on SERENITY_ROOT as
set in the CMakeLists.txt. *Requiring* the user to set it doesn't make
sense anymore.
While I was in there anyway, I added exit code propagation. Also, 'find' should
be a tad faster now, because it doesn't enumerate files in the large ignored
directories Build/ and Toolchain/ anymore.