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/0x.js/CHANGELOG.json

@@ -7,7 +7,8 @@
                 "pr": 1102
             },
             {
-                "note": "Added `MetamaskSubprovider` to handle inconsistencies with signing.",
+                "note":
+                    "Added `MetamaskSubprovider` to handle inconsistencies in Metamask's signing JSON RPC endpoints.",
                 "pr": 1102
             },
             {

packages/0x.js/src/index.ts

@@ -90,6 +90,7 @@ export {
     JSONRPCRequestPayload,
     JSONRPCResponsePayload,
     JSONRPCErrorCallback,
+    JSONRPCResponseError,
     LogEntry,
     DecodedLogArgs,
     LogEntryEvent,