fixed: error "Error detected while processing function <SNR><N>_detect"

Some files can be unreadable by a current user when traversing up
through "neibor"-files. If so, the following error occurs:
  Error detected while processing function <SNR><N>_detect:
  line   12:
  E484: Can't open file <unreadable_filename>

To avoid this error, let's be sure we can read that file before we try
to read it.