feat: clarify missing file error message

[goller]

The error returned from buildkit was confusing.
This simplifies the error message to the useful
details.

Previous:

 => ERROR [5/5] COPY howdy /doody                                                                                     0.0s
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 > [5/5] COPY howdy /doody:
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Error: failed to solve: failed to compute cache key: failed to calculate checksum of ref vv3yzwr47shug36z88iqkt7yq::l6be4rvphai7t4cl6oyb2j8mu: "/howdy": not found

Now:

 => ERROR [5/5] COPY howdy /doody                                                                                     0.0s
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 > [5/5] COPY howdy /doody:
------

Error:  "/howdy": not found. Please check if the files exist in the context.

[jacobwgillespie]

Nice 💥

I'm torn between what you have here, where we replace the confusing error with the friendly one, or something where we add the friendly interpretation before or after the existing message, in case somebody's familiar with BuildKit errors.

At the moment, I think what you have makes sense to me, because idk if anybody is actually depending on the original error message, and the new one is way clearer