cli: if response code from wapi server is not 200 then stop instead of proceeding

Also, include the data that failed to json parse in an exception raised by the json parser.

src/allmydata/scripts/tahoe_ls.py

@@ -30,13 +30,22 @@ def list(options):
         print >>stderr, "Error during GET: %s %s %s" % (resp.status,
                                                         resp.reason,
                                                         resp.read())
+        if resp.status == 0:
+            return 3
+        else:
+            return resp.status
+
     data = resp.read()
 
     if options['json']:
         print >>stdout, data
         return
 
-    parsed = simplejson.loads(data)
+    try:
+        parsed = simplejson.loads(data)
+    except Exception, le:
+        le.args = tuple(le.args + (data,))
+        raise
     nodetype, d = parsed
     children = {}
     if nodetype == "dirnode":