[WIP] Fix faulty error serialization logic

[alexfertel]

Hello!

I am trying to fix https://github.com/MetaMask/eth-rpc-errors/issues/51, so I tried to follow what @rekmarks said on the issue. However, I don't think it solves what https://github.com/MetaMask/eth-rpc-errors/issues/48 is about. I couldn't find a test covering the edge case, so it would be nice if you gave some pointers on how to address the error mangling.

I'll be more than happy to help fix it!