fix formatting of ValueError if horizon_elevation is less than 0

[sjoerd-bouma]

Tiny bugfix - the error emitted if horizon_elevation < 0 (added in #25) was not formatted correctly. Weirdly, this seems not to cause any issues on Python >= 3.8 (other than presumably raising a different error instead of raising the ValueError), but it seems Python <= 3.7 complains already at compile time, which breaks the module.

Okay, I was not familiar with this - apparently this is annotated string formatting, which was added only with Python 3.8. So this is actually just another compatibility fix for outdated Python versions : )

[what-the-diff]

PR Summary

• Improved Error Message Presentation in Horizon Elevation Validation
  • The error message display has been updated to provide a clearer presentation of "current horizon elevation" calculation.