Make dirty checking in check_gn_headers.py more robust

build/check_gn_headers.py

Index: build/check_gn_headers.py
diff --git a/build/check_gn_headers.py b/build/check_gn_headers.py
index 1db587a7d2ef2d403d0226281b28838a9721d379..f6ae8f5cac4166221e2dc1108050a4502a9daadd 100755
--- a/build/check_gn_headers.py
+++ b/build/check_gn_headers.py
@@ -148,9 +148,12 @@ def GetDepsPrefixes(q):
 
 def IsBuildClean(out_dir):
   cmd = [os.path.join(DEPOT_TOOLS_DIR, 'ninja'), '-C', out_dir, '-n']
-  out = subprocess.check_output(cmd)
-  return 'no work to do.' in out
-
+  try:
+    out = subprocess.check_output(cmd)
+    return 'no work to do.' in out
+  except Exception as e:
+    print e
+    return False
 
 def ParseWhiteList(whitelist):
   out = set()