when to create a new exception code example

Example: Where did you manually create an exception?

In Driver Class. Basically I am getting Browser Name 
from ConfiguratinProperty. If some invalid browser names 
are giving in that propertyfiles is your task is it still 
gonna run? Is it gonna throw exception? It is going to throw 
exception message such as Nosuchbrowser. In the Driver Clas 
is none of the names are match with valid browsers than I can 
Throw this exception manually.