allow stdin fix to exit sucessfully

[PierreAntoineGuillaume]

When submitting a file to phpcbf from stdin, phpcbf always fails.

I propose to make it succeed (exit 0) when no errors were found, given that cbf has no direct concept of "non-fixable errors".

It would allow tools to swap content of a buffered file when exit code is non-zero, and discard if nothing has been fixed.

[gsherwood]

Changing a return value would need to target a new major version (4.0 in this case), and that is already being tracked on issue #2898.

Thanks for taking the time to write this PR, but I'm going to close it as a proper plan of the exit codes need to be made so they all make sense together. If you have a proposal, please drop it on that other issue to keep the discussion going.