Use lua_pcall() for mud.descriptor.on_command().

(And check the return values of lua_pcall() against LUA_OK.)

src/descriptor.c: Use LUA_OK.
src/lua_descriptor.c: Use lua_pcall().
src/lua_timer.c: Use LUA_OK.
src/lua_zmq.c: Use LUA_OK.
Use LUA_OK when checking the return value of lua_resume().

src/lua_descriptor.c: Do it.