Add error messages for BlitFramebuffer related errors These are particularly helpful when debugging multiview rendering that requires a framebuffer blit. Existing blitFramebuffer error message strings are moved to ErrorStrings.h. BUG=angleproject:1617 TEST=angle_end2end_tests Change-Id: I6e8b45355045d80abf044714ac4b9d818c53bf46 Reviewed-on: https://chromium-review.googlesource.com/1175125 Commit-Queue: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Jamie Madill <jmadill@chromium.org>