Errors not bubbling up from BatchJob

[kgilden]

This is related to https://github.com/symfony/assetic-bundle/issues/350.

I'm not an expert in how forking exactly works, but I think there might be a problem with BatchJob silently failing, if debug is set to false. In that case forks won't throw exceptions and promises must be used (this part here). However, there seems to be no error handling done in BatchJob. If a fork failed for any reason, the Fork::getResult() would return null and a PHP warning would be generated.

PHP Warning:  array_merge(): Argument #2 is not an array in /project/vendor/kriswallsmith/spork/src/Spork/Batch/BatchJob.php on line 96

Since BatchJob itself runs in a fork (i.e. we have [main process] <-> [batch job] <-> [n child processes]), wouldn't it make sense to throw an exception, if any of the child forks failed? Here's one way to do it, but hopefully more knowledgeable devs could take a look into it and propose a better fix.