Better error message for wrong import implementation

[rylev]

This changes improve the error message when a component is importing/exporting something that the linker is either missing or has the wrong type.

Previously the error message for a missing function implementation would be:

Error: import `my-import` has the wrong type

Caused by:
    expected func found nothing

Now, this error will be:

Error: component imports `my-import`, but a correct implementation of `my-import` could not be found

Caused by:
    function implementation is missing

I went back and forth on whether the error message should reference the linker. For direct users of Linker this would surely help, but this error message also gets displayed to indirect users of Linker so talking at a higher level of abstraction seemed appropriate.

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[peterhuene]

I think what you have is definitely trending in the right direction for an easier to understand error message.

Just a few feedback comments.

Additionally, "implementation" in some of the errors rubs me the wrong way for some reason I can't seem to articulate, nor can I think of a better word to use.

[rylev]

@peterhuene I believe I've addressed your feedback. I agree that "implementation" isn't always the right word, but I think we'll need to let this sit for a while before the right word makes itself apparent.