Def of var with a ns that doesn't exist will yield this error:
Cause: Compiler.lookupVar() returns null if the ns in a qualified var does not exist yet.
Proposed: The error message would be improved by naming the symbol and throwing a CompilerException with file/line/col info. It's not obvious, but this may be the only case where this error occurs. If so, the error message could be more specific that the ns is the part that doesn't exist.