BUGFIX: Streamline exit(1) to web response with HTTP status code 500

[christophlehmann]

With this critical error messages like the following now sends the HTTP status code 500 instead 200 in order to state something is not okay on the server side:

Flow could not create the directory "/var/www/html/Data/Persistent". Please check the file permissions manually or run "sudo ./flow flow:core:setfilepermissions" to fix the problem. (Error #1347526553)

Resolved: #3364

Checklist

• [x] Code follows the PSR-2 coding style
• [ ] Tests have been created, run and adjusted as needed
• [ ] The PR is created against the lowest maintained branch
• [x] Reviewer - PR Title is brief but complete and starts with FEATURE|TASK|BUGFIX
• [x] Reviewer - The first section explains the change briefly for change-logs
• [ ] ~Reviewer - Breaking Changes are marked with !!! and have upgrade-instructions~

[kitsunet]

I would merge this in 24h if no one else has a look here.

[kitsunet]

This technically belongs into 8.3 unless there are strong reasons for it to go somewhere else, as everythign else is in security fix only mode already. I would do that on merge then.