how to handle the error in transaction sql

Create Proc TranTest
AS
BEGIN TRAN

INSERT INTO [authors]([au_id],
[au_lname],
[au_fname],
[phone],
[contract])
VALUES ('172-32-1176',
'Gates',
'Bill',
'800-BUY-MSFT',
1)

-- First Update query
UPDATE authors
SET au_fname = 'Johnzzz'
WHERE au_id = '172'

-- second Update query
UPDATE authors
SET au_fname = 'Johnzzz'
WHERE au_id = '175'

--third Update query
UPDATE authors
SET au_fname = 'Johnzzz'
WHERE au_id = '178'

COMMIT TRAN
GO

in the above i written three update statement, if suppose in the second update statment gets error, whether the first update statement record gets updated.

for that how to handle using transact sql