test/py: Raise a ValueError if a command fails

At present an Exception is raised if a command fails. This is a very broad
class and makes it difficult for callers to catch the error without also
catching other things, like programming bugs.

Change it to ValueError to make this easier.

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

test/py/multiplexed_log.py

@@ -165,7 +165,7 @@ class RunAndLog(object):
         if output and not output.endswith('\n'):
             output += '\n'
         if exit_status and not exception and not ignore_errors:
-            exception = Exception('Exit code: ' + str(exit_status))
+            exception = ValueError('Exit code: ' + str(exit_status))
         if exception:
             output += str(exception) + '\n'
         self.logfile.write(self, output)