Increase verbosity of "not readable" error message

While setting up Thruk, I got an error message saying "cgi.cfg not readable", whereas the file was there (as "ssi/cgi.cfg") and perfectly readable.
I changed the error message to make sure the script was effectively trying to read "ssi/cgi.cfg", but it turned out that it was looking for "cgi.cfg" one level higher (Adding a symlink fixed it !)