Is this a single user application? If not, how do you plan to keep the global temp table name unique for the thread that is creating it?

Using global temp tables for error handling is a bad idea. Why do you think you need to use a temp table to pass error information back to the calling SP?

BOL has a bunch of detailed examples of using TRY/CATCH blocks to process errors. I suggest you read them. You're going to get yourself into a world of hurt trying to use global temp tables to do this.