Do not return a no-op function if default function isn't found

[AlexEne]

This PR changes the linker.get_default() deafult return value. It removes the return of a no-op function as this is rarely what users of the method would want. I have replaced it with an error that indicates that the module has no default functions to return.

This has been briefly discussed in the zulip bytecode alliance channel.

I do not know how to add test cases to this, it seems that there were no tests around it, but I'm happy to add some if anyone could guide me on where to put them.

I am unsure who the right reviewer for this is.

[AlexEne]

It seems I have missed some tests, right now the following are failing:

    cli_tests::greeter_preload_command
    cli_tests::minimal_reactor

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[abrown]

Maybe @alexcrichton?

[alexcrichton]

I've requested review from @sunfishcode for this

[AlexEne]

@sunfishcode if you're ok with this change I'll go ahead and fix the tests.

[sunfishcode]

This change looks good to me. This is a space that we're still figuring out as we go; I think making it an error now makes sense.

[AlexEne]

Ok, I'll fix the tests and update this PR in the next days

[bjorn3]

This will need to be updated for #2897. @AlexEne do you want to do this or should I take it over?

[AlexEne]

Oh wow, I forgot about this. So sorry. I would be able to pick it up again later next week, but I'm swamped with other stuff so if you want to land this it may be faster to do it.

It's not a massive change and waiting for me isn't really necessary.

[bjorn3]

I don't have any immediate need for this. I was just triaging the PR queue a bit.

[AlexEne]

Ok, I've set up some time to update this PR on Friday next week.

[AlexEne]

Closing this and will re-open a correct one on the latest version