server:run command: provide more error information

The server:run command didn't provide many information when the executed command exited unexpectedly. Now, the process' exit code is passed through and an error message is displayed.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

This is, for example, useful, if you provide an invalid ip address/hostname on which the server couldn't listen.
In this case, the error output would also be shown if the process was terminated by something like ``kill`` (the exit code then is 143). Do we consider that to be an unexpected termination? If so, I'll agree with you and will change the code accordingly.