Improved error message to explicitly state allowable characters
Improved error message to explicitly state allowable characters

Related to Bug #6432.

brunostein/pfsense,ptorsten/pfsense,ptorsten/pfsense,phil-davis/pfsense,pfsense/pfsense,pfsense/pfsense,dennypage/pfsense,ptorsten/pfsense,jxmx/pfsense,NewEraCracker/pfsense,pfsense/pfsense,dennypage/pfsense,phil-davis/pfsense,phil-davis/pfsense,NewEraCracker/pfsense,NewEraCracker/pfsense,dennypage/pfsense,phil-davis/pfsense,brunostein/pfsense,jxmx/pfsense,brunostein/pfsense,jxmx/pfsense,NOYB/pfsense,NOYB/pfsense,NOYB/pfsense,pfsense/pfsense,NewEraCracker/pfsense,dennypage/pfsense