Don't bother with a backtrace when we produce certain fatal errors.

We'll introduce a new "Fatal" exception for this purpose, and throw it when
we just want to print a user message and abort immediately.

helpers.py

@@ -15,3 +15,7 @@ def debug1(s):
 def debug2(s):
     if verbose >= 2:
         log(s)
+
+
+class Fatal(Exception):
+    pass