run_command: log error messages using higher log level when return code is non-zero

run_command logs stdout and stderr output using the "debug" level. In
this patch, if returncode is non-zero, it logs stderr output using the
"error" level. It's helpful for diagnosing underlying command errors.

Signed-off-by: Zhou Zheng Sheng <zhshzhou@linux.vnet.ibm.com>