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>
diff --git a/tools/buildman/control.py b/tools/buildman/control.py
index b86d7b3..0b6ab03 100644
--- a/tools/buildman/control.py
+++ b/tools/buildman/control.py
@@ -237,7 +237,7 @@
         options.step = len(series.commits) - 1
 
     gnu_make = command.Output(os.path.join(options.git,
-                                           'scripts/show-gnu-make')).rstrip()
+            'scripts/show-gnu-make'), raise_on_error=False).rstrip()
     if not gnu_make:
         sys.exit('GNU Make not found')