binman: Output errors to stderr

At present binman outputs errors to stdout which means that fails are
effectively silent when printed by buildman, for example. Fix this by
outputing errors to stderr.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

tools/binman/main.py

@@ -123,7 +123,7 @@ def RunBinman(args):
         try:
             ret_code = control.Binman(args)
         except Exception as e:
-            print('binman: %s' % e)
+            print('binman: %s' % e, file=sys.stderr)
             if args.debug:
                 print()
                 traceback.print_exc()