patman: Convert 'Series-xxx' tag errors into warnings

If the Series-xxx tag is not recognised patman currently reports a fatal
error. This is inconvenient if a new feature is later added to patman that
an earlier version does not support.

Report a warning instead, to allow the user to take action if needed, but
still allow operation to proceed.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/patman/patchstream.py

@@ -114,7 +114,9 @@ class PatchStream:
             self.in_section = name
             self.skip_blank = False
         if self.is_log:
-            self.series.AddTag(self.commit, line, name, value)
+            warn = self.series.AddTag(self.commit, line, name, value)
+            if warn:
+                self.commit.warn.append(warn)
 
     def _add_to_commit(self, name):
         """Add a new Commit-xxx tag.