Bug #1601630: little improvement to getopt docs

(backport from rev. 52833)

Doc/lib/libgetopt.tex

@@ -126,8 +126,9 @@ import getopt, sys
 def main():
     try:
         opts, args = getopt.getopt(sys.argv[1:], "ho:v", ["help", "output="])
-    except getopt.GetoptError:
+    except getopt.GetoptError, err:
         # print help information and exit:
+        print str(err) # will print something like "option -a not recognized"
         usage()
         sys.exit(2)
     output = None
@@ -135,11 +136,13 @@ def main():
     for o, a in opts:
         if o == "-v":
             verbose = True
-        if o in ("-h", "--help"):
+        elif o in ("-h", "--help"):
             usage()
             sys.exit()
-        if o in ("-o", "--output"):
+        elif o in ("-o", "--output"):
             output = a
+        else:
+            assert False, "unhandled option"
     # ...
 
 if __name__ == "__main__":