Merge branch 'refactor/check-revert-reasons' into feature/combinatorial-testing

* refactor/check-revert-reasons:
  Temporarily switch revert reasons to `TransferFailed`. Should be `InvalidAmount` but because of an oversight in the assembly implementation of `dispatchTransferFrom`, it always throws `TransferFailed`
  Expect RevertReason be passed in, not string
  Rename RevertReasons to RevertReason since singular enum names are more common

# Conflicts:
#	packages/contracts/test/asset_proxy/proxies.ts
#	packages/contracts/test/exchange/core.ts

packages/contracts/src/utils/assertions.ts

@@ -1,3 +1,4 @@
+import { RevertReason } from '@0xproject/types';
 import * as chai from 'chai';
 import * as _ from 'lodash';
 
@@ -59,7 +60,10 @@ export function expectRevertOrAlwaysFailingTransactionAsync<T>(p: Promise<T>): P
  * @returns a new Promise which will reject if the conditions are not met and
  * otherwise resolve with no value.
  */
-export function expectRevertReasonOrAlwaysFailingTransactionAsync<T>(p: Promise<T>, reason: string): PromiseLike<void> {
+export function expectRevertReasonOrAlwaysFailingTransactionAsync<T>(
+    p: Promise<T>,
+    reason: RevertReason,
+): PromiseLike<void> {
     return _expectEitherErrorAsync(p, 'always failing transaction', reason);
 }