Supressed error when shell_exec is disallowed

[troendheim]

When shell_exec has been disabled through the "disable_functions" php-setting, the run-method silently fails and sets pid = 0.

This happens as you cast to integer on the following line:
https://github.com/cocur/background-process/blob/master/src/BackgroundProcess.php#L74

shell_exec returns null when an error occurs - (int) null === 0.
A RuntimeException should be thrown if shell_exec returns null.