SUSHI should report a more helpful error message when a referenced Extension is missing a snapshot

The error message provided is not clear as to what is missing or why. We may be able to improve the error message to be more helpful.

One confusing piece of this is that the example provided in the FSH Online link does not produce consistent error messages between FSH Online in Chrome, FSH Online in Firefox, and with SUSHI in the terminal. They are all similar, so it may be the same cause, but we should investigate and see if we can improve the error.

See: https://chat.fhir.org/#narrow/stream/215610-shorthand/topic/Error.20on.20external.20Extension.20without.20Snapshot

FSH Online link: https://fshschool.org/FSHOnline/#/share/3thVPDo

Reproduce the shared FSH Online example in Chrome and Firefox, then compare its output with SUSHI in the terminal. Use the linked chat discussion for context; done means the error identifies what is missing and why, with consistent behavior across these cases.

Written by the indexing model from the issue text.