Setting/modifiying CSSUnitValue.unit should be consistent on Error thrown

Author: Naina Raisinghani

css-typed-om/Overview.bs

@@ -1421,7 +1421,7 @@ are represented as {{CSSUnitValue}}s.
     perform the following steps:
 
     1. If [=creating a type=] from |unit| returns failure,
-        [=throw=] a {{SyntaxError}}
+        [=throw=] a {{TypeError}}
         and abort this algorithm.
 
     2. Return a new {{CSSUnitValue}}