Improve 'EventSource()' exception messages.

Rather than throwing the generic 'SecurityError' and 'SyntaxError' text,
this patch adds information about the reasons behind the throw in order
to improve the developer experience.

BUG=152212,270041
R=arv@chromium.org

Review URL: https://codereview.chromium.org/22690002

git-svn-id: svn://svn.chromium.org/blink/trunk@155839 bbb929c8-8fbe-4397-9dbb-9b2b20218538