Here is an example that I just tried and it works

DECLARE @x XML(CustomerSchema), @y XML

SELECT @y = 'a'

BEGIN TRY

SELECT @x = @y

END TRY

BEGIN CATCH

RAISERROR('Hey, this is an invalid XML',16,1)

END CATCH

/*

OUTPUT:

Msg 50000, Level 16, State 1, Line 7

Hey, this is an invalid XML

*/