Don't lose NSError code in RCTJSErrorFromNSError()

Summary:Fixes facebook#6171

Send original error code back to JS - this reverts old functionality according to facebook#6171 (comment), that is https://github.com/facebook/react-native/blob/3c541ca54095fbf9d572362bcb661593acc1b64b/React/Base/RCTUtils.m#L299
Closes facebook#6201

Differential Revision: D2988618

Pulled By: javache

fb-gh-sync-id: 85a280b76e557bffc4d952ed7f39eeb8a88b66ba
shipit-source-id: 85a280b76e557bffc4d952ed7f39eeb8a88b66ba

React/Base/RCTUtils.m

@@ -372,7 +372,8 @@ BOOL RCTClassOverridesInstanceMethod(Class cls, SEL selector)
 
 NSDictionary<NSString *, id> *RCTJSErrorFromNSError(NSError *error)
 {
-  return RCTJSErrorFromCodeMessageAndNSError(RCTErrorUnspecified,
+  NSString *codeWithDomain = [NSString stringWithFormat:@"E%@%zd", error.domain.uppercaseString, error.code];
+  return RCTJSErrorFromCodeMessageAndNSError(codeWithDomain,
                                              error.localizedDescription,
                                              error);
 }