Throw immediately for poor “map” input.

[theengineear]

Currently, when you pass a non-array value to map… it doesn’t yell at you. Only when you go to actually go to bind the value that value do you get an error.

The problem is that iff we immediately throw an error, the actual stack trace will point you to the line-in-question. However, if we wait for later to do this… we lose that context — which is a major loss of development efficiency.