UTIL_getFileSize() returns UTIL_FILESIZE_UNKNOWN on failure

UTIL_getFileSize() returns UTIL_FILESIZE_UNKNOWN on failure

UTIL_getFileSize() used to return zero on failure.
This made it impossible to distinguish a failure from a genuine empty file.
Both cases where coalesced.

Adding UTIL_FILESIZE_UNKNOWN constant has many consequences on user code,
since in many places, the `0` was assumed to mean "error".
This is no longer the case, and the error code must be actively checked.