rebaseline.py: add --keep-going-on-failure option, off by default

tools/rebaseline_imagefiles.py

Index: tools/rebaseline_imagefiles.py
===================================================================
--- tools/rebaseline_imagefiles.py	(revision 10046)
+++ tools/rebaseline_imagefiles.py	(working copy)
@@ -278,15 +278,8 @@
                     continue
             outfilename = os.path.join(self._expectations_root, subdir,
                                        filename);
-            # TODO(epoger): Until we resolve
-            # https://code.google.com/p/skia/issues/detail?id=1410 ('some GM
-            # result images not available for download from Google Storage'),
-            # keep going in the face of missing results for any one test.
-            try:
-                self._RebaselineOneFile(expectations_subdir=subdir,
-                                        builder_name=builder,
-                                        infilename=filename,
-                                        outfilename=outfilename,
-                                        all_results=all_results)
-            except Exception as e:
-                print 'WARNING: swallowing exception %s' % e
+            self._RebaselineOneFile(expectations_subdir=subdir,
+                                    builder_name=builder,
+                                    infilename=filename,
+                                    outfilename=outfilename,
+                                    all_results=all_results)