Conditionally use AC's ObservableQuery.resetQueryStoreErrors

To fix issue #3090, the `ObservableQuery.resetQueryStoreErrors`
method was introduced in `apollo-client` 2.6.3. While
`apollo-client` 2.6.3 is a peer dep of the latest version of
`react-apollo` (2.5.7), people who can't update their version
of `apollo-client` to >= 2.6.3 are running into issues when
updating to the latest version of `react-apollo`, since
`ObservableQuery.resetQueryStoreErrors` isn't available to them.
Since we can't enforce the version of `apollo-client` people are
using, this commit adjusts the `Query` component to only use
`resetQueryStoreErrors` if it's available. If it isn't, it will
call into the `ObservableQuery`'s private API to do the same
things as `resetQueryStoreErrors`. This is a hack, but it is
temporary as `react-apollo` is launching soon, and will enforce
a minimum `apollo-client` version of 2.6.3 (so this workaround
won't be needed).

Fixes #3148.

src/Query.tsx

@@ -488,8 +488,27 @@ export default class Query<TData = any, TVariables = OperationVariables> extends
     // remove those errors from the `ObservableQuery` query store, so they
     // aren't re-displayed on subsequent (potentially error free)
     // requests/responses.
+    //
+    // NOTE: Resetting query store errors is handled in 2 different ways here,
+    // since the `resetQueryStoreErrors` wasn't available until
+    // `apollo-client` 2.6.3. If a previous version of `apollo-client` is
+    // being used, errors are reset by reaching into `ObservableQuery`'s
+    // internals. This hack is temporary, as React Apollo 3 will be
+    // released shortly, and will enforce `apollo-client` 2.6.3 as the
+    // minimum.
     setTimeout(() => {
-      this.queryObservable!.resetQueryStoreErrors();
+      if ((this.queryObservable! as any).resetQueryStoreErrors) {
+        // Apollo Client >= 2.6.3
+        (this.queryObservable! as any).resetQueryStoreErrors();
+      } else {
+        // Apollo Client < 2.6.3
+        const { queryManager, queryId } = (this.queryObservable! as any);
+        const queryStore = queryManager.queryStore.get(queryId);
+        if (queryStore) {
+          queryStore.networkError = null;
+          queryStore.graphQLErrors = [];
+        }
+      }
     });
 
     result.client = this.client;