FHIRPath evaluator should throw when string function is invoked on non-string

[lmsurpre]

Describe the bug
http://hl7.org/fhirpath/N1/#string-manipulation says this about the String functions in this section:

If there is more than one item, or an item that is not a String, the evaluation of the expression will end and signal an error to the calling environment.

However, users report that our engine returns empty in this case instead of throwing.

Environment
4.11.1

To Reproduce
See https://chat.fhir.org/#narrow/stream/179266-fhirpath/topic/error.20handling

Expected behavior
"the evaluation of the expression will end and signal an error to the calling environment"

Additional context

[lmsurpre]

I tried the happy path on various data types including:

• Resource.id, Element.id, and Extension.url (System.String)
• Extension.value.as(FHIR.string)
• Extension.value.as(FHIR.uri)
• Extension.value.as(FHIR.code)
• Extension.value.as(FHIR.oid)
• Extension.value.as(FHIR.uuid)
• Extension.value.as(FHIR.markdown)
• Extension.value.as(FHIR.base64Binary)

Interestingly, I found that the implementation for oid and uuid also match values with the prefix (e.g. "id" will always match the "oid" or "uuid" in the prefix, regardless of the actual unique values).
Similarly, I found that the base64Binary variant matches the encoded base64Binary string, not the decoded value.
I have no clue if that is 'correct' or not, but it seems reasonable and so I"m moving on.

I also confirmed the negative cases for

• more than one item in the input collection
• an input collection of size 1 where the type is not string