Throw and handle errors from Providers.

In web3 wrapper when a response contains an error field we throw this rather than return response.result which is often undefined.
In Signature Utils we handle the error thrown when a user rejects the signing dialogue to prevent double signing.
Exposed the ZeroExTransaction JSON schema.
In Website only use the MetamaskSubprovider if we can detect the provider is Metamask

packages/web3-wrapper/test/web3_wrapper_test.ts

@@ -1,5 +1,5 @@
 import * as chai from 'chai';
-import { BlockParamLiteral } from 'ethereum-types';
+import { BlockParamLiteral, JSONRPCErrorCallback, JSONRPCRequestPayload } from 'ethereum-types';
 import * as Ganache from 'ganache-core';
 import * as _ from 'lodash';
 import 'mocha';
@@ -78,6 +78,19 @@ describe('Web3Wrapper tests', () => {
             const signatureLength = 132;
             expect(signature.length).to.be.equal(signatureLength);
         });
+        it('should throw if the provider returns an error', async () => {
+            const message = '0xdeadbeef';
+            const signer = addresses[1];
+            const fakeProvider = {
+                async sendAsync(payload: JSONRPCRequestPayload, callback: JSONRPCErrorCallback): Promise<void> {
+                    callback(new Error('User denied message signature'));
+                },
+            };
+            const errorWeb3Wrapper = new Web3Wrapper(fakeProvider);
+            expect(errorWeb3Wrapper.signMessageAsync(signer, message)).to.be.rejectedWith(
+                'User denied message signature',
+            );
+        });
     });
     describe('#getBlockNumberAsync', () => {
         it('get block number', async () => {