executeCommandInProcess should throw if the command fails

[LeBenLeBen]

The BaseAction->executeCommandInProcess() should throw an Exception in the same way as CommandAction->execute() does.

[jeanmonod]

@LeBenLeBen Could have been a way of doing things. But right now the $process is returned and it's the caller responsibility to do something with it. You can see an example here: https://github.com/liip/RMT/blob/master/src/Liip/RMT/Prerequisite/TestsCheck.php#L47

I'm not doing to change that now, as it will create a BC break. Sorry