confmgt: include variable name in all complaints.
When we give a msg about failing to parse a variable, we should say what the variable was, and ideally what was wrong with it.
Compare the results for running
./src/app/tor UseBridges 99 in 0.3.5 and in master. With 0.3.5 you got:
Boolean 'UseBridges 99' expects 0 or 1. but now you get
Unrecognized value 99.
Let's make that better.