NativeFunctionCall prints enum values in error messages, not value type strings

[furkle]

Overview

I have noticed multiple times that runtime error messages produced by inkjs differ in slight but significant ways by those outputting by Inky. In particular, when one attempts to perform a logical operation like > on a variable which is not a number, the error message one should get looks approximately like this:

Cannot perform operation > on String

However, in inkjs, this looks like so:

Cannot perform operation > on 3

After a colleague pointed out this was likely referring to a type, not a value, we found that error messages from NativeFunctionCall.ts were including the value from the enum, which is an integer, rather than the key, which is a string.

Reproduction

1. Produce a runtime error by comparing two variables which are of an incomparable type.
2. Note the error produced in inkjs differs from that produced by Inky.

Pull request

#479

[ephread]

Oh, that's another nice find @furkle, thanks! I'll have a look at the PR.

[furkle]

Travis is failing so unfortunately I'm not sure if it's actually a working PR, but it's basically a two-line fix, so I hope it works 😄