improve error message for >=1.1 forecast

[peterdudfield]

Verification

• I have searched the existing issues to ensure this feature has not already been requested.

Provide a detailed description of the proposed feature.

If we try to add a forecast that is over 1.1 of the capacity, then we get a funny error that says smallint is too big.

It would be good if we get this error beforehand

What is the motivation for the feature?

we have got this error in live, and it was a bit confusing to debug

Outline a potential approach or solution (optional).

No response