Use better error message for an HTMLMediaElement that has already been connected.

The new error message makes it clear why createMediaElementSource is
throwing an error instead of the previous generic "invalid".

Also simplify the layout test implementation to also show the errors
thrown.

No functional changes.

BUG=429204

Review URL: https://codereview.chromium.org/692213004

git-svn-id: svn://svn.chromium.org/blink/trunk@184797 bbb929c8-8fbe-4397-9dbb-9b2b20218538