Specify what happens with exponential or interval property function type mismatches

[jfirebaugh]

For example, what is the result of evaluating the following function against a feature whose p property value is not numeric?

{
  "property": "p",
  "stops": [
    [0, 0],
    [10, 100]
  ]
}

cc @lucaswoj

[lucaswoj]

Currently this is undefined behaviour.

If we're looking to define the behaviour in this case, I'd suggest implementing a default value per #480 and using that value in cases like this.

[lucaswoj]

This issue was moved to mapbox/mapbox-gl-js#4146