Improve HTMLTableRowElement exception messages.

This CL cleans up the exception messages for.insertCell() and .deleteCell().
It also removes an exception thrown when setting .cells. While this attribute
is 'readonly', we don't generally throw exceptions, but just ignore the set.
This matches Firefox's behavior for the attribute.

As a drive-by, two setter methods are removed which were never defined.

BUG=270033

Review URL: https://codereview.chromium.org/83213002

git-svn-id: svn://svn.chromium.org/blink/trunk@162546 bbb929c8-8fbe-4397-9dbb-9b2b20218538