Revert "Add backtrace to error function"

This reverts commit 5b518ed2feff28c9bf21ad940c1b211b72d71bd1.

This caused duplicate tracebacks and tracebacks when unwanted.
It also ignored the level argument to error() and didn't pass the message (or level) to debug.traceback().
Use xpcall() or lua_pcall()'s errorhandler argument instead.
Update documentation on is_ground_content