FrameCapture: Add more helpful texture errors Rather than asserting, print an error message that provides some helpful details to guide capture triage. Test: Fortnite MEC Bug: b/180418810 Bug: angleproject:5658 Change-Id: I03ee2a12fa1185698650643f8767cd758bd87a4d Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3105387 Commit-Queue: Cody Northrop <cnorthrop@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Tim Van Patten <timvp@google.com>