Error message when creating camp with overlapping sub-camps not user-friendly

[Joelius300]

When creating a camp with multiple sub-camps that overlap, this is the error message you get. This has caused confusion for our coach. Additionally, this validation only happens when submitting, so at a point when the input fields that cause the error aren't even in view anymore.

[carlobeltrame]

For completeness' sake, the reasons that make this validation necessary in the first place are tracked in #2454