systemctl: improve readability on failed commands

Not long ago a failed command would print:
"Failed to start something.service: ..."
regardless of whether the command was to start/stop/restart/etc.

With e3e0314 this was improved to print the method used. E.g. for stopping:
"Failed to StopUnit something.service: ..."

This patch matches the method to a more human readable word. E.g:
"Failed to stop something.service: ..."