grit: fix various undefined variable errors

Bug: 983071
Test: `./grit/test_suite_all.py` passes
Change-Id: Ia2f519670a280042c60616ae961a1d10b6d8ed25
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1698110Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676661}

tools/grit/grit/gather/skeleton_gatherer.py

@@ -12,6 +12,7 @@ import types
 
 from grit.gather import interface
 from grit import clique
+from grit import exception
 from grit import tclib
 
 

tools/grit/grit/grd_reader.py

@@ -9,6 +9,7 @@
 from __future__ import print_function
 
 import os.path
+import sys
 import types
 import xml.sax
 import xml.sax.handler

tools/grit/grit/node/misc.py

@@ -293,9 +293,6 @@ class ReleaseNode(base.Node):
   def DefaultAttributes(self):
     return { 'allow_pseudo' : 'true' }
 
-  def GetReleaseNumber():
-    """Returns the sequence number of this release."""
-    return self.attribs['seq']
 
 class GritNode(base.Node):
   """The <grit> root element."""