json.extractAt should report which field failed and why #1

When using extractAt to parse a struct from JSON, if a field can't be parsed (e.g., type mismatch), the function returns null with no indication of what went wrong.

Reproduction: Try parsing a JSON object with "action": "create" into a struct with action: SomeEnum. The extraction silently fails.

Suggested improvement: Return or log diagnostic info like:

extractAt failed: field 'action' - expected enum SomeEnum, got string "create"

This would save significant debugging time when working with external JSON APIs (like TAP) where the exact field types aren't always documented.