Remove reserved exit code 255 and replace it with exit code 1

[PrinsFrank]

Exit code 255 is reserved by PHP: https://www.php.net/manual/en/function.exit.php

Note: Exit codes should be in the range 0 to 254, the exit code 255 is reserved by PHP and should not be used.

While there is no definitive standard on exit codes, 255 is also generally reserved, see the linux documentation project (unofficial source)

This PR removes all usages of exit 255 and replaces it with exit code 1, in line with the changes in #110 where exit code 1 was already used for errors in the JUnitFormatter. Using exit codes 0 and 1 is also in line with symfony console and tools like composer and PHPStan.

[janedbal]

Seems valid, thank you!