[Process] Fix #5594 : `termsig` must be used instead of `stopsig` in exceptions when a process is signaled

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #5594
| License       | MIT

When a process is signaled, an exception is thrown saying `The process has been signaled with signal "0".`. This is wrong because "0" is not a signal.

This is actually a bug in `Process` because we retrieve the `stopsig` property of the process status whereas we should retrieve the `termsig`.

`stopsig` should be retrieved in case the process has been stopped, and `termsig` when it has been signaled, see [PHP documentation](http://www.php.net/manual/en/function.proc-get-status.php).