Improve error message when referencing unknown properties

[curran]

Currently, when a property is referenced but not defined, the following error occurs:

reactive-model-v0.8.0.min.js:1 Uncaught TypeError: Cannot read property 'id' of undefined

Tasks:

• Create a failing test that reproduces the error.
• Implement code that detects when this error will occur, and throw an error that says (e.g. for property "foo") The property "foo" was referenced as a dependency for a reactive function before it was defined. Please define each property before referencing them in reactive functions.
• Check for this text in the unit test

[curran]

Part of reactive-model 1.0 #31

[curran]

Part of #33

[curran]

This should actually be solved in reactive-function. Created a new issue over there datavis-tech/reactive-function#20