internal/screentest: log screenshot errors

Adds log output for screenshot errors so they visible
before all testcases complete.

Change-Id: Ida229f256164e16d28fdeedf3671012976659bf3
Reviewed-on: https://go-review.googlesource.com/c/website/+/379135
Run-TryBot: Jamal Carvalho <jamal@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Trust: Jamal Carvalho <jamalcarvalho@google.com>

internal/screentest/screentest.go

@@ -638,6 +638,7 @@ func (tc *testcase) run(ctx context.Context, update bool) (err error) {
 		return nil
 	})
 	if err := g.Wait(); err != nil {
+		fmt.Fprint(&tc.output, err)
 		return err
 	}
 	result := imgdiff.Diff(*screenA, *screenB, &imgdiff.Options{