Improve 'Selection' exception messages.

Rather than throwing generic exceptions, we should give developers a bit
of context around what failed and why. This patch addresses that concern
for the Selection APIs.

BUG=270033

Review URL: https://chromiumcodereview.appspot.com/22608004

git-svn-id: svn://svn.chromium.org/blink/trunk@156131 bbb929c8-8fbe-4397-9dbb-9b2b20218538