Improve WebGL SecurityError exception messages.

We're currently writing unsuppressable console messages for
'getImageData', and have only generic error messages for 'texImage2D' and
'texSubImage2D'. This patch improves both.

BUG=152212

Review URL: https://chromiumcodereview.appspot.com/23058009

git-svn-id: svn://svn.chromium.org/blink/trunk@156304 bbb929c8-8fbe-4397-9dbb-9b2b20218538