This is the first step to simplify the signals emitted by CliInterface.
The failOperation() function is used all over the place, while in fact is
just calling doKill(). But after commit 074d527, doKill() doesn't emit anymore finished().

So we fix this by renaming it to a function that actually kills the process and
that may or may not cause the finished() signal to be emitted, depending on the context.