Exception raised when negative parking rates are entered

Client crash reported by Cryptog in a topic on https://discuss.nubits.com. This affects rates outside of the range of normal range that would be entered so it likely isn't a critical fix and would rarely be encountered (only by accident), but just in case it's indicative of another deeper problem it should be reviewed and fixed when it's convenient.

Version Affected: 1.2.0 OS(s) Affected: Awaiting response from reporter