Remove exception related methods from ScriptState

Now that setException() is unused, so we can remove it. The fact that we can remove setException() means that we can also remove exception(), hadException() and clearException(). This CL does the clean-up.

BUG=357144

Review URL: https://codereview.chromium.org/226013002

git-svn-id: svn://svn.chromium.org/blink/trunk@170938 bbb929c8-8fbe-4397-9dbb-9b2b20218538