update nested VList warning to error

Summary:
This has caused SEVs because the warning has gone unnoticed, so upgrading to error which should be much harder to miss or ignore.

https://fb.workplace.com/groups/rn.core/permalink/2680043542227367/

# Changelog
[Internal] update nested VList warning to error

Reviewed By: TheSavior

Differential Revision: D21945364

fbshipit-source-id: 88a9a9ab0b51e0afcf9b25be9854f65a61f419af

Libraries/Lists/VirtualizedList.js

@@ -1112,9 +1112,10 @@ class VirtualizedList extends React.PureComponent<Props, State> {
               this.context == null
             ) {
               // TODO (T46547044): use React.warn once 16.9 is sync'd: https://github.com/facebook/react/pull/15170
-              console.warn(
+              console.error(
                 'VirtualizedLists should never be nested inside plain ScrollViews with the same ' +
-                  'orientation - use another VirtualizedList-backed container instead.',
+                  'orientation because it can break windowing and other functionality - use another ' +
+                  'VirtualizedList-backed container instead.',
               );
               this._hasWarned.nesting = true;
             }