How to use TO_NUMBER function to update columns of a table Using oracle pl/sql
As per your comment, requesting a regex solution: 

update table1 set 
  error_code = 1
, error_desc = 'Incorrect format' 
where not regexp_like( cust_no, '^[0-9]+$' )


However, I would suggest that this is too little, too late. 

You should not be allowing this defective data into the database in the first place!  (Unless this is part of a migration exercise and this is a transitional table). 

Your application should be detecting this bad formatting and not even saving the record to the database. 

Second Thought: 

Alternatively, put the check into the table definition 

[Air Code; untested]

create table Table1
( Cust_No Varchar2(50) 
, CHECK( regexp_like( Cust_No '^[0-9]+$' )
. . . 


Third Thought: 

If you customer number is, indeed, a number, then why are you storing it in the wrong Data Type? 

create table Table1
( Cust_No  NUMBER 
. . .