API ioMapping ignores typos, should error instead

[DDR0]

Example:

cam-json -n set - <<< '{"ioMapping": {"io1": {"threshold": "1.5"}}}'

Expected Behaviour:
io1 is set to a trigger threshold of one-and-a-half volts.

Actual Behaviour:
io1 is not touched, nor is any error raised

The Actual Problem:
It is bleedingly hard to find the typo, sometimes. Unused keys should raise a warning, at least, and preferably an error. This makes the API much easier to use.