Improve exception messages for SVGAngle and SVGLength.

Also add some additional tests for SVGLength to capture one of the affected exception messages.

BUG=270033

Review URL: https://codereview.chromium.org/106543007

git-svn-id: svn://svn.chromium.org/blink/trunk@164277 bbb929c8-8fbe-4397-9dbb-9b2b20218538