improve VirtualizedList error message (#25973)

Summary:
Motivation: when you receive error like `scrollToIndex out of range: 5 vs -1` it's not immediately clear if I requested 5 or -1. This will make the error a little easier to understand.

## Changelog

not needed
Pull Request resolved: https://github.com/facebook/react-native/pull/25973

Test Plan: not needed, tests must pass

Differential Revision: D16708522

Pulled By: osdnk

fbshipit-source-id: 8dfcbd95ff0f42805dbe32cd57969a93aea55add

Libraries/Lists/VirtualizedList.js

@@ -338,7 +338,9 @@ class VirtualizedList extends React.PureComponent<Props, State> {
     const {animated, index, viewOffset, viewPosition} = params;
     invariant(
       index >= 0 && index < getItemCount(data),
-      `scrollToIndex out of range: ${index} vs ${getItemCount(data) - 1}`,
+      `scrollToIndex out of range: requested index ${index} but maximum is ${getItemCount(
+        data,
+      ) - 1}`,
     );
     if (!getItemLayout && index > this._highestMeasuredFrameIndex) {
       invariant(