Insert Stored Procedure Doesn't Create Database Record

I have the following stored procedure:

ALTER PROCEDURE Pro_members_Insert
@id int outPut,
@LoginName nvarchar(50),
@Password nvarchar(15),
@FirstName nvarchar(100),
@LastName nvarchar(100),
@signupDate smalldatetime,
@Company nvarchar(100),
@Phone nvarchar(50),
@Email nvarchar(150),
@Address nvarchar(255),
@PostalCode nvarchar(10),
@State_Province nvarchar(100),
@City nvarchar(50),
@countryCode nvarchar(4),
@active bit,
@activationCode nvarchar(50)
AS
declare @usName as varchar(50)
set @usName=''
select @usName=isnull(LoginName,'') from members where LoginName=@LoginName
if @usName <> ''
begin
set @ID=-3
RAISERROR('User Already exist.', 16, 1)
return
end
set @usName=''
select @usName=isnull(email,'') from members where Email=@Email
if @usName <> ''
begin
set @ID=-4
RAISERROR('Email Already exist.', 16, 1)
return
end
declare @MemID as int
select @memID=isnull(max(ID),0)+1 from members
INSERT INTO members (................................

When I run this page, signup.aspx, provide required fields and click submit, the page simply reloads and the database table does not reflect the newly-inserted record. How do I catch the error messages that might be returned from the sproc? how to change signup.aspx so that the insert occurs.