Throw a warning/error on non-existant variable interpolation

[wynro]

I have been stuck in a lesson for a while because the bootdev client was sending the following header to my server:

Authorization: Bearer ${jwtTokenMike}

(Literally, without interpolating the variable)

The issue was that I made a mistake in a previous check and the variable wasn't being filled so the interpolation didn't work, so it was my mistake.

I think it would be better if instead of silently ignoring the non-existant variable the client showed a warning or an error of some kind to make the error more obvious.