chore(idFromString): make error message friendlier

[thiskevinwang]

Description

The error alone is a bit cryptic, and if you're seeing it, the solution is somewhat buried in docs: https://developers.cloudflare.com/durable-objects/best-practices/access-durable-objects-from-a-worker/#parse-previously-created-ids-from-strings

This makes the idFromString error slightly friendlier