add some more error checks into _fileExistsInDirectory function to report "too long name" error

Hi,
In another thread[1], Álvaro gave some feedback for _fileExistsInDirectory function for "too long name" error.
Basically, in _fileExistsInDirectory function, we pass dirname and filename but we were checking only the combined length of these two names.

Here, I am attaching a patch which will check lengths of dirname and filename separately and will report errors if the name is too long.

I added a check in some other parts also to report an error for "too long name".

Please review the attached patch and let me know feedback.

[1]: https://www.postgresql.org/message-id/202504110938.4kx73ylnv6p4%40alvherre.pgsql