Add notes on backwards incompatibility in error cases in WebGL 2

As in GLES, behavior in the cases where an earlier API version generates
an error is subject to change in future API versions. Also, in some cases
GLES3.0 can sample a texture even though GLES2.0 would return RGBA =
0, 0, 0, 1.

It seems like a good idea to mention this kind of things in the backwards
incompatibility section for the sake of exhaustiveness, though a complete
list of changed error conditions would seem excessive.
Clarify texture access differences in WebGL 2

The only rule that WebGL 2 lifts is that accessing NPOT textures with
certain wrapping or filtering modes should return 0,0,0,1. Other rules
concerning accessing incomplete textures still apply. Make this clear.
Merge pull request #749 from NVIDIA/backwards-incompatibility-errors-note

Add notes on backwards incompatibility in error cases in WebGL 2