patman: Adjust command.Output() to raise an error by default

It is more useful to have this method raise an error when something goes
wrong. Make this the default and adjust the few callers that don't want to
use it this way.

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

tools/patman/gitutil.py

@@ -391,7 +391,8 @@ def EmailPatches(series, cover_fname, args, dry_run, raise_on_error, cc_fname,
     """
     to = BuildEmailList(series.get('to'), '--to', alias, raise_on_error)
     if not to:
-        git_config_to = command.Output('git', 'config', 'sendemail.to')
+        git_config_to = command.Output('git', 'config', 'sendemail.to',
+                                       raise_on_error=False)
         if not git_config_to:
             print ("No recipient.\n"
                    "Please add something like this to a commit\n"