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>