Fix RPC error messages

[rekmarks]

Fix error message issues described in: #7138 (comment)

9/18/2019 update: Ready to review.

Pending

• Fix error serialization json-rpc-engine#19

In a previous commit, json-rpc-engine started using eth-json-rpc-errors for error serialization. This error serialization swallowed the original error message if the error had a missing or invalid code. The above PR to json-rpc-engine preserves the original error message if it exists and adds a valid code (JSON RPC Internal error, -32603) if the original code was missing/invalid.

With that change and the code changes introduced in this PR, all RPC errors returned from the extension to the inpage provider will follow JSON RPC 2.0 and EIP 1193, allowing dapp devs to identify and handle errors by code. Consistently using the eth-json-rpc-errors API internally will allow us to do the same.

[Gudahtt]

Looks good! It works for me locally.

[Gudahtt]

I suspect this closes #7160

[tmashuang]

I didn't even see #7160. Seems that it fixed the provider approval error, but not the signing methods they mentioned.
Is this where the error handling should go in their respective files?

metamask-extension/app/scripts/lib/message-manager.js

Lines 84 to 87 in 4d88e1c

	case 'rejected':
	return reject(new Error('MetaMask Message Signature: User denied message signature.'))
	default:
	return reject(new Error(`MetaMask Message Signature: Unknown problem: ${JSON.stringify(msgParams)}`))

metamask-extension/app/scripts/lib/personal-message-manager.js

Lines 90 to 93 in 7985f4f

	case 'rejected':
	return reject(new Error('MetaMask Message Signature: User denied message signature.'))
	default:
	return reject(new Error(`MetaMask Message Signature: Unknown problem: ${JSON.stringify(msgParams)}`))

metamask-extension/app/scripts/lib/typed-message-manager.js

Lines 80 to 85 in cc71b4f

	case 'rejected':
	return reject(new Error('MetaMask Message Signature: User denied message signature.'))
	case 'errored':
	return reject(new Error(`MetaMask Message Signature: ${data.error}`))
	default:
	return reject(new Error(`MetaMask Message Signature: Unknown problem: ${JSON.stringify(msgParams)}`))

[danfinlay]

I noticed one typo which is arguably breaking.

I don't fully understand how eth-json-rpc-errors improves error messages, or how this fixes a former break, so it might be nice to also document those two things, even just here in the PR before approve/merge.

[danfinlay]

Looks good, thanks for adding the extended explanation!