Improve error output when a contentReference element won't unfold

[mint-thompson]

Transferred from internal JIRA issue originally created 2023-06-12.

In ElementDefinition.unfold, errors may occur if the element specified in a contentReference can’t be found. Currently, the errors are language-level, such as Cannot read properties of undefined (reading 'length') rather than SUSHI-specific messages. Update this function so that more useful error messages are produced.

This was found in the HL7/davinci-epdx repository, which uses USCore 3.1.1 as a dependency. The following FSH will cause an error with an unhelpful message:

Profile:        ProblemProvenance
Parent:         us-core-provenance
* entity.agent.role = #12345

The contentReference on the entity.agent element will not resolve, since it refers to a BackboneElement that does not use the profile element extension.