Make bad numberstrings error instead of warn

[qaisjp]

See:

• 51a2c78 "Fix bad numberstrings becoming 0", and
• 73692fd "Make bad numberstrings warn instead of error"

This warning has been around since 1.5.3. I propose we make this an error in 1.6.

Please discuss and express your (dis)approval.

[Pirulax]

I totally agree, there's no reason you would accept a bad number string, the user can explicitly check if its a bad numberstring or not. This may just cause misbehaving.

Also, Im pretty sure its rare you have a bad numberstring, but even if you do, you dont want it to modify anything into a possibly incorrect state.