patman: Make exception handling python 3.x safe

Syntax for exception handling is a little more strict in python 3.x.
Convert all uses to a form accepted by both python 2.x & python 3.x.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Acked-by: Simon Glass <sjg@chromium.org>

tools/patman/command.py

@@ -85,7 +85,7 @@ def RunPipe(pipe_list, infile=None, outfile=None,
 
         try:
             last_pipe = cros_subprocess.Popen(cmd, cwd=cwd, **kwargs)
-        except Exception, err:
+        except Exception as err:
             result.exception = err
             if raise_on_error:
                 raise Exception("Error running '%s': %s" % (user_pipestr, str))