validate delay(referencedNode, ..., best_guess) in node summary

[tedchwang]

Is your feature request related to a problem? Please describe.
When a formula contains a delay that uses "best_guess" as the initial value, we should validate whether the referenced node contains a "latest value". Otherwise, calcWorker may give cryptic errors like: TypeError: Unexpected type of argument in function multiplyScalar (expected: number or Complex or BigNumber or Fraction or Unit or string or boolean, actual: undefined, index: 0)

Describe the solution you'd like
It'd be nice to have validation when the formula is entered.

Describe alternatives you've considered
It's also possible to validate during a simulation run, but it should be better to help users detect this type of error early.