lagoon.Fill now returns a *lagoon.FillTypeError naming the key when a
requested value cannot be stored in its column (a fraction, exponent or
overflow for an integer field, or a value of the wrong type). The admin
save path maps it to a validation_failed 422 on that field instead of a
500 CapabilityError; genuine capability failures keep the 500.
- lagoon.Fill converts a json.Number (from a UseNumber decoder, as cabana's
save path uses) into integer, unsigned and float fields; a fraction or an
overflow is an error
- before this, saving a type: number field into an *int column was a 500
- README documents the conversion
