Syntax checking DO blocks and ALTER TABLE statements?

How does one go about syntax checking this?

do $$
begin if exists (select 1 from information_schema.table_constraints
       where constraint_name = 'error_to_web_service_error') then
            raise notice 'EXISTS error_to_web_service_error';
    else
        ALTER TABLE web_service_error
           ADD CONSTRAINT error_to_web_service_error FOREIGN KEY (error_id)
           REFERENCES error_code(error_id)
           ON DELETE NO ACTION NOT DEFERRABLE INITIALLY IMMEDIATE;
    end if
end $$

(There are 222 ALTER TABLE ADD FOREIGN KEY statements that I'm wrapping in
similar DO blocks, and want to make sure the statements are clean.)

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