after user enters formula, validate that variables and custom functions exist

[tedchwang]

Is your feature request related to a problem? Please describe.
When errors of this type are discovered at time of baseline calculation, the user may not have the context and short-term memory to fix the error easily.

Describe the solution you'd like
It'll be nice to try compiling the formula after the user enters it, and report error right away.

Describe alternatives you've considered
An alternative is to leave the error to be discovered at simulation run time, which is less helpful to users.