More descriptive error message when bundle already defined

[Ambient-Impact]

After trying to track down some errors in a project, I noticed LoadJS just throws "LoadJS" without any additional information as to what caused it. This outputs the bundle ID that is already defined (which causes the error to be thrown) in a more descriptive error message.

[amorey]

A more descriptive error message would be very helpful but the promise we made with this project is to keep the number of bytes down as much as humanly possible and this change adds 26 bytes to the gzipped output. Is there a way we can make the error more descriptive while keeping the byte count down?

[Ambient-Impact]

I hadn't considered that. It might be possible to reduce the size with clever use of this or that, but it's not something I have much experience with. I'm mostly coming at this with the aim to save other people some frustration. I suppose you could just output the bundle ID alone and that would be an improvement and hopefully not change the size with gzipping?

[amorey]

Outputting the bundle ID sounds good to me. It will probably actually decrease the bundle size. Do you want to make the change in the PR?

[Ambient-Impact]

Sorry that took so long - been busy with a lot of other stuff.