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/buildman/control.py

@@ -237,7 +237,7 @@ def DoBuildman(options, args, toolchains=None, make_func=None, boards=None,
         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')