Make the type validator outputs better

**Describe the bug**
The ontology validator does not help the user to understand build failures because of the way it presents errors to the user.
For example:

`ERROR : Unit "meters_per_square_second" has the unknown measurement type "linearacceleration"`

What is the error? Is it with meters_per_square_second or with linear_acceleration? Pretty unclear. It also gives no guidance about what to fix.

Instead of raising the unhelpful error above, the validator should output something more like this:

`ERROR : Subfield 'linear_acceleration' is undefined, but the unit "meters_per_square_second" references it. Define the subfield or assign the unit to another subfield.`

This sort of user-friendly error message should be added everywhere, for everything.