Summary:
Explain the **motivation** for making this change. What existing problem does the pull request solve?
See https://github.com/facebook/react-native/issues/10506. A native `NSError` with `NSUnderlyingErrorKey` set causes a JSON stringify error from the websocket dispatcher if remote debugging is enabled.
**Test plan (required)**
I'm not familiar with the react native testing framework. Happy to add a test for this if someone can point me to where this part of the codebase is exercised :)
I did some spot checks with nil user dictionaries and nil underlying errors here. The case that this solves is testable using https://github.com/superseriouscompany/react-native-error-repro, specifically:
```objective-c
NSError *underlyingError = [NSError errorWithDomain:@"underlyingDomain" code:421 userInfo:nil];
NSError *err = [NSError errorWithDomain:@"domain" code:68 userInfo:@{@"NSUnderlyingError": underlyingError}];
reject(@"foo", @"bar", err);
```
Closes https://github.com/facebook/react-native/pull/10507
Differential Revision: D4080802
Pulled By: lacker
fbshipit-source-id: 93a41d9e9a710e406a6ccac214a5617271b4bede
c144bbfb7e