Print plain text error message rather than error code

[Olivier-ProGlove]

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.

[nvlsianpu]

@Olivier-ProGlove I believe this should be reported to https://github.com/apache/mynewt-mcumgr (which is pointed by zephyr-project documentation). Issue is probably valid here as well - but can you open issue in repository i mentioned?

[Olivier-ProGlove]

Done: https://github.com/apache/mynewt-mcumgr/issues/10