fix handling of KV set and throw wrapped errors

[karthik2804]

Fixes a couple of small bugs that I ran into.

• We were not handling the case ArrayBuffer properly in KV set operations.
• The responses returned by starlingMonkey were cyclic, and routers like Itty threw errors when trying to go from an error to a response. This fix now extracts the useful parts of the error and wraps them up in a simpler JS error.

[karthik2804]

@tschneidereit I am not really sure about the error part and wonder if handling the error should be a user concern instead of us wrapping it.

[tschneidereit]

I think that's an okay workaround, yes. It's somewhat unfortunate that the generated bindings have cyclic structures, but we should look into tackling that with the move to wit-dylib in time.

[tschneidereit]

Tiniest of nits: I would move this to the top so the encode cases stay grouped