Improve error message when you forget to parameterize a call with [cpp_xxx]

[gordonwatts]

• Detect the user is trying to call soemthing that isn't a method/function, and print an error message with context.

Fixes #160

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.44%. Comparing base (8f97702) to head (31e217b).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #185   +/-   ##
=======================================
  Coverage   97.43%   97.44%           
=======================================
  Files          15       15           
  Lines        1367     1369    +2     
=======================================
+ Hits         1332     1334    +2     
  Misses         35       35           

Flag	Coverage Δ
unittests	97.44% <100.00%> (+<0.01%)	⬆️

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