ggml : improve error handling for search path existence checks (llama/17653)

ggml : improve error handling for search path existence checks (llama/17653)

* Improve error handling for search path existence checks

Refactor existence checks for search paths using std::error_code to handle potential errors.

* Improve cache file existence check with error code

Update fs::exists to use std::error_code for error handling.

* Simplify existence check for search paths

Simplify existence check for search paths

* Fix logging path in error message for posix_stat

* Update ggml/src/ggml-backend-reg.cpp

Co-authored-by: Aman Gupta <redacted>
* Adapt to the coding standard

---------

Co-authored-by: Aman Gupta <redacted>