[Debug] Check headers sent before sending PHP response

If the response contents has been sent before an error occurs, PHP triggers the warning "Cannot modify header information - headers already sent".

This change ensure that the useful error message is echoed, while it's impossible to change the HTTP status code and headers.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a