jsii kernel gives quite unhelpful error message when serializing a function

[rix0rrr]

🐛 Bug Report

Now obviously, functions should never get passed to the jsii kernel to serialize, so this is already a bug. But if they do:

jsii.errors.JavaScriptError: 
  Error: Resolution error: JSII kernel assumption violated, undefined is not an object.

Upon reading this error message you'll tear out your hair trying to figure out how this is possible, because no code path looks like it should allow undefined at that point.

This is because:

> JSON.stringify(function() { return 666; })
undefined

Thanks JavaScript!

Anyway, explicitly testing for functions using typeof value === 'function' and throwing a more appropriate error message would be VERY VERY helpful.

[RomainMuller]

The error message was improved & now provides a suggestion as per a probable cause, too.