Print plain text error message rather than error code

I recently got `Error: 1` while I was expected my `mcumgr` command line to work. See my issue report here: https://github.com/zephyrproject-rtos/zephyr/issues/7924#issuecomment-407478098

Luckily @nvlsianpu pointed me the statement in the documentation saying this error was expected:

> Note that if you try to send the very same image that is already flashed in slot-0 then the procedure will not complete successfully since the hash values for both slots will be identical.

I would not have this issue if `mcumgr` would have returned a plain text error message.