bail early on errors

Returning errors can feel warm and fuzzy, but it does no good if the
caller doesn't actually check the return value.

Scenario:
  define a new function B
  call it when initializing global variable A
  everything works fine
  restart

Before this commit you get a weird error because we silently stopped
loading, and now lots of functions are missing.

After this commit you get a better error message.

live.lua

@@ -79,7 +79,7 @@ function live.load_all()
     assert(buf and buf ~= '')
     local status, err = live.eval(buf)
     if not status then
-      return err
+      error(err)
     end
   end
 end