[Process] fixed fatal errors in getOutput and getErrorOutput when process was not started
[Process] Trow exceptions in case a Process method is supposed to be called after termination
bug #10480 [Process] Fixed fatal errors in getOutput and getErrorOutput when process was not started  (romainneutron)

This PR was merged into the 2.3 branch.

Discussion
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[Process] Fixed fatal errors in getOutput and getErrorOutput when process was not started

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10022
| License       | MIT

This PR replaces #9452 and address the latest changes.
Side note : I've not updated `getExitCode`, `getExitCodeText` and `isSuccessful` as they were explicitly tested to return null in case the process is not started or terminated. I think it would be a BC break to do that.

Commits
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449fe01 [Process] Trow exceptions in case a Process method is supposed to be called after termination
0ae6858 [Process] fixed fatal errors in getOutput and getErrorOutput when process was not started