Another way might be to modify /usr/share/backuppc/lib/BackupPC/CGI/GeneralInfo.pm in a way that it produces a useable error message if there is no /usr/bin/rrdtool binary.
Another way might be to modify /usr/share/ backuppc/ lib/BackupPC/ CGI/GeneralInfo .pm in a way that it produces a useable error message if there is no /usr/bin/rrdtool binary.