Fix the reference money formatter for a refactor

An AI agent replaced a hand-rolled money formatter with a new Money type across 120 call sites in your billing code, and called the change behavior-preserving. To check that claim, you run the same amounts through the old path, the new path and one reference formatter, and compare the strings.

An assistant wrote format_money(amount_minor, currency), the reference formatter. It is in the editor. It has a bug. Find it and fix it.

Input and output:

• amount_minor: an amount in the currency's minor unit, such as cents. It can be negative.
• currency: an ISO 4217 currency code, such as USD.
• Return the formatted amount, or ERR.

Rules the tests check:

• How many minor units each currency has, which is the number of digits after the decimal point:
  • 2: USD, EUR, GBP, CHF, CAD, AUD
  • 0: JPY, KRW, VND
  • 3: KWD, BHD, JOD
• Any other code returns ERR. Codes match exactly, so usd is not USD.
• Otherwise return the code, one space, a - if the amount is negative, then the amount without its sign in major units.
• The major units are the whole part, a ., then exactly as many fraction digits as the currency has minor units, padded with leading zeros. With 0 minor units, write the whole amount with no ..
• No thousands separators.