allow configuring behavior on unsupported conversion

[NQNStudios]

Here you said:

I think this is fine as long as you don't need the value back (and obviously you don't because you just store it as null)
On the other hand I think a proper fix is to introduce some runtime metadata such that the conversion behaviour can be controlled by user code.

This was right--there are a lot of cases where you might want to suppress conversion errors. My other PR #8 is failing tests because of one of those.

So this PR reverts the special-case handling of the Lua class in #7 and instead introduces a variable that configures handling for every unsupported conversion.

[kevinresol]

LGTM, thanks!