Use NGX_FILE_ERROR for handling file operations errors.

On Win32 platforms 0 is used to indicate errors in file operations, so
comparing against -1 is not portable.

This was not much of an issue in patched code, since only ngx_fd_info() test
is actually reachable on Win32 and in worst case it might result in bogus
error log entry.

Patch by Piotr Sikora.