clamonacc: add logging to watchpoint hierarchy add

[m-sola]

Patch provides more insight into error conditions which may arise when adding a directory to the watch hierarchy. If a specific file caused the issue, the filename is provided to help users with any troubleshooting needed.

[val-ms]

There are also some places where err (a cl_error_t return code) is printed directly. E.g.

            logg(LOGG_ERROR, "ClamInotif: could not parse include list (%d)\n", err);

It would probably help make if we switch to cl_strerror, like:

            logg(LOGG_ERROR, "ClamInotif: could not parse include list (%s)\n", cl_strerror(err));