Scope of exception in plsql is handled by Exception Block.. 
this is depends on the code you had written.
suppose u have created one more plsql block inside a plssql 
block,if the inner block is not handled any exception then 
the outer block will handle the exception by defining the 
situation/even like "when others then /  when value_error 
then... etc "

Note.. when u r using "when Others then" all the exceptions 
will handle by this exception.. Even there is no error the 
plsql will return 0 for this exception that is executed by 
default in a plsql block. So. be carefull while writting 
this kind of exception ("When others then")

Raising an error in PL/SQL execution block is nothing 
but "Execption". These are  of 2 types.
1.Predefined Exception 2. User defined Exception

Total 19+1 predefined exceptions are there.
19+1 means 19+When Others.
"When others" is  the last one.After "When Others" no 
exception will be raised.
Eg. for Predefined Exceptions: 
No_data_found,Dup_val_on_index,Invalid_cursor,Invalid_number
,Zero_Divide,.......,'When Others'..

Thank you,
Suresh.