you could place GO statements between each command, but you still have to manually ignore the errors...makes it hard to determine which are "ignorable" errors and which are real.

instead, consider checking for the object before you create it:

IF NOT EXISTS(select 1 from sys.columns where name='Field1' and object_name(object_id) = 'Table1')

ALTER TABLE Table1 ADD [Field1] nvarchar(4);

IF NOT EXISTS(select 1 from sys.columns where name='Field2' and object_name(object_id) = 'Table1')

ALTER TABLE Table1 ADD [Field2] nvarchar(2);

IF NOT EXISTS(select * from sys.objects where name='Foo' and type_desc = 'PRIMARY_KEY_CONSTRAINT' and object_name(parent_object_id) = 'Table2')

ALTER TABLE Table2 ADD CONSTRAINT Foo PRIMARY KEY ([ID])

IF NOT EXISTS(select 1 from sys.columns where name='Field1' and object_name(object_id) = 'Table3')

ALTER TABLE Table1 ADD [Field1] nvarchar(50);