Where to Trap error...?

[jyaseena]

Table Name : Jobs

Jobid Minsal Maxsal
J1 1000 2000
J2 2001 3000
J3 3001 4000

I have to update the column Minsal and maxsal . there is a condition Maximum salary should be greater than minimumsalary.
if user break this rule obiviously it will show error.

we have two set of codeing is here . one is in backend and second is in front end.
in front end its very easy trap the two field(Minsal & maxsal) whether its less than minsal . it will show error.

but in the case of backend we write it as below :
Exception
when sal_error then
Raise_application_error(-20001,'Data error...Max salary should be greater than min salary...');

here there are 2 questions is that why we trap this error in back end ? and anyway if you write it in backend how we will pass this error to frond end..?

As I am a oracle beginer ... expect a good replay