No load error on at macroexpand

[bramtayl]

There's three reasons I think this would be a good idea:

1. Semantic: code that is macroexpanded is not loaded, at least, it's certainly not evaluated.

2. Not particularly useful: load errors contain a line number and module, but that's not particularly helpful for macroexpand, which is usually used interactively

3. Convenience: not having to unwrap loaderrors to test macros is more convenient. I think a strong case for this can be made just by looking at the tests I've changed that no longer need to do this.

[bramtayl]

Also worth noting that it's my understanding that errors can be changed for minor versions

[bramtayl]

Bump?

[bramtayl]

Bump?

[bramtayl]

Bump??

[JeffBezanson]

I agree. Rebase this and we can merge it.

[c42f]

I agree with removing LoadError!

However, note that this will break package tests spuriously because packages commonly test their macros with code like

@test_throws LoadError macroexpand(some_module, :(some_ex))

In my earlier effort to remove LoadError more widely (see #31882) I had a solution to this by grandfathering in LoadError in @test_throws handling.

[bramtayl]

@c42f oh I didn't realize there was previous work on this. Do you think it's worth it to try to do a deprecation? Any comments on whether the previous implementation was better/worse? (I barely knew enough C to do this PR).

[c42f]

A few thoughts:

• It's worth reading the discussion on that PR for some background. Jeff's comment at Soft deprecation of LoadError in runtime #31882 is relevant.
• I probably tried to do too much in that PR which is why it got stuck. Especially all the changes I was trying to make to interpreter backtraces were quite involved. So you can ignore most of the C code there.

For this PR, I think it's very worth taking the relevant code from the first commit of #31882 (see 3ee9d8f). Especially the changes to the Base tests and to the Test stdlib in do_test_throws, etc. This will prevent a lot of needless breakage in packages which currently depend on LoadError being thrown.

[bramtayl]

Ok, I've included the deprecation and rebased as #38379