Use different exit code when desired dep roll is a no-op

roll_dep.py

Index: roll_dep.py
diff --git a/roll_dep.py b/roll_dep.py
index f6db7971d2688a284fe91372907d5bb6ff55f8bd..10c58295659a33c75b1d48fe7b39f45a0aeed7d4 100755
--- a/roll_dep.py
+++ b/roll_dep.py
@@ -22,6 +22,10 @@ class Error(Exception):
   pass
 
 
+class AlreadyRolledError(Error):
+  pass
+
+
 def check_output(*args, **kwargs):
   """subprocess.check_output() passing shell=True on Windows for git."""
   kwargs.setdefault('shell', NEED_SHELL)
@@ -109,7 +113,7 @@ def roll(root, deps_dir, roll_to, key, reviewers, bug, no_log, log_limit,
   print('Found new revision %s' % roll_to)
 
   if roll_to == head:
-    raise Error('No revision to roll!')
+    raise AlreadyRolledError('No revision to roll!')
 
   commit_range = '%s..%s' % (head[:9], roll_to[:9])
 
@@ -213,7 +217,7 @@ def main():
 
   except Error as e:
     sys.stderr.write('error: %s\n' % e)
-    return 1
+    return 2 if isinstance(e, AlreadyRolledError) else 1
 
   return 0