Require Range to be a range in UTFStringCodePoints to improve error messages

[eggrobin]

Suggested by @jmr after seeing the error messages generated by UTFStringCodePoints<..., char> (which was how this API worked in an earlier draft).

TODO: Fill out the checklist below.

Checklist

• Required: Issue filed: ICU-NNNNN
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

[eggrobin]

@markusicu, should this target main on a new ticket, or 78 maintenance on a reopened ICU-23004?

[markusicu]

@markusicu, should this target main on a new ticket, or 78 maintenance on a reopened ICU-23004?

@eggrobin is this PR fixing problems, or just "improv[ing] error messages"?
If it's an important bug fix, then it should go onto the maintenance branch, with the existing ticket.
If it's merely an improvement, then it should go onto main for ICU 79.

@mihnita @richgillam FYI

[markusicu]

lgtm as an improvement for ICU 79 -- please create an ICU 79 ticket for such improvements