Fix TS promise error types in web3-eth

[cgewecke]

The pattern used in that file looks like:

https://github.com/ethereum/web3.js/blob/3e10668745ddbff8c785351f36754343ec680acf/packages/web3-eth/types/index.d.ts#L295-L298

This passes Web3's types tests but as reported in comment #3248 comment, TS complains when making an assignment like:

// Does not compile
const a: TransactionReceipt = await sendTransaction(...) 

// This is ok though..
const a = await sendTransaction(...) 

There's a guide to defining Promise error types here.