fix: improve error handling

[wemeetagain]

Motivation

• UnhandledPromiseRejection in sendPing #265
• Error: ENR has no udp multiaddr lodestar#6320

Notes

• refactor callback handling
  • move from callback style (err, resp) => void to deferred promise style {resolve: (err) => void, reject: (err) => void}
  • ensure callbacks are called in each rpc handler and in each error case
• remove hanging promises
  • add linter rule to ensure nothing remaining
• move error handling of node contacts into getNodeContact rather than the getNodeAddress "getter"
  • this makes error handling much more predictable, throwing upfront, rather than waiting for a later event to trigger an error
• wrap rpc handlers in try/catch statements
  • ensure that logic handed in rpc request and response handlers are unable to trigger unhandled exceptions

[nflaig]

lgtm