Minor edits
Minor edits

Fixes for a few minor issues discovered while auditing usage of String().

- Error constructors should only ignore message if undefined, not other falsey values.

- Add comment about possibly polyfilling Error subclass constructors.

- Formatting and whitespace fixes.

google/CodeCity,NeilFraser/CodeCity,NeilFraser/CodeCity,google/CodeCity,google/CodeCity,NeilFraser/CodeCity,NeilFraser/CodeCity,google/CodeCity