Allow serializing underlying NSError objects, closes #10506

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

Examples/UIExplorer/UIExplorerUnitTests/RCTJSONTests.m

@@ -42,6 +42,25 @@
   XCTAssertEqualObjects(json, RCTJSONStringify(text, NULL));
 }
 
+- (void)testEncodingNSError
+{
+  NSError *underlyingError = [NSError errorWithDomain:@"underlyingDomain" code:421 userInfo:nil];
+  NSError *err = [NSError errorWithDomain:@"domain" code:68 userInfo:@{@"NSUnderlyingError": underlyingError}];
+
+  // An assertion on the full object would be too brittle since it contains an iOS stack trace
+  // so we are relying on the behavior of RCTJSONParse, which is tested below.
+  NSDictionary<NSString *, id> *jsonObject = RCTJSErrorFromNSError(err);
+  NSString *jsonString = RCTJSONStringify(jsonObject, NULL);
+  NSDictionary<NSString *, id> *json = RCTJSONParse(jsonString, NULL);
+  XCTAssertEqualObjects(json[@"code"], @"EDOMAIN68");
+  XCTAssertEqualObjects(json[@"message"], @"The operation couldn’t be completed. (domain error 68.)");
+  XCTAssertEqualObjects(json[@"domain"], @"domain");
+  XCTAssertEqualObjects(json[@"userInfo"][@"NSUnderlyingError"][@"code"], @"421");
+  XCTAssertEqualObjects(json[@"userInfo"][@"NSUnderlyingError"][@"message"], @"underlying error");
+  XCTAssertEqualObjects(json[@"userInfo"][@"NSUnderlyingError"][@"domain"], @"underlyingDomain");
+}
+
+
 - (void)testDecodingObject
 {
   NSDictionary<NSString *, id> *obj = @{@"foo": @"bar"};

React/Base/RCTUtils.m

@@ -413,18 +413,28 @@ NSDictionary<NSString *, id> *RCTJSErrorFromCodeMessageAndNSError(NSString *code
 {
   NSString *errorMessage;
   NSArray<NSString *> *stackTrace = [NSThread callStackSymbols];
+  NSMutableDictionary *userInfo;
   NSMutableDictionary<NSString *, id> *errorInfo =
   [NSMutableDictionary dictionaryWithObject:stackTrace forKey:@"nativeStackIOS"];
 
   if (error) {
     errorMessage = error.localizedDescription ?: @"Unknown error from a native module";
     errorInfo[@"domain"] = error.domain ?: RCTErrorDomain;
+    if (error.userInfo) {
+      userInfo = [error.userInfo mutableCopy];
+      if (userInfo != nil && userInfo[NSUnderlyingErrorKey] != nil) {
+        NSError *underlyingError = error.userInfo[NSUnderlyingErrorKey];
+        NSString *underlyingCode = [NSString stringWithFormat:@"%d", (int)underlyingError.code];
+        userInfo[NSUnderlyingErrorKey] = RCTJSErrorFromCodeMessageAndNSError(underlyingCode, @"underlying error", underlyingError);
+      }
+    }
   } else {
     errorMessage = @"Unknown error from a native module";
     errorInfo[@"domain"] = RCTErrorDomain;
+    userInfo = nil;
   }
   errorInfo[@"code"] = code ?: RCTErrorUnspecified;
-  errorInfo[@"userInfo"] = RCTNullIfNil(error.userInfo);
+  errorInfo[@"userInfo"] = RCTNullIfNil(userInfo);
 
   // Allow for explicit overriding of the error message
   errorMessage = message ?: errorMessage;