Shouldn't allow negative payments or credits

Overview

Negative amounts in payments or credits change the meaning of the transaction and should not be allowed.

Expected

A payment of a negative amount should be rejected
A credit of a negative amount should be rejected

Actual

both Payments and Credits can be successfully posted with negative values.

See the comments of MODFIN-71 for examples.

Acceptance Criteria

• Negative payments/credits are disallowed
• An appropriate error message is returned
• unit tests are updated
• API tests are updated