Throws: oldIE native Error types, remove String

Handle oldIE native Error types in `throws`.
Remove the String comparison type from `throws`.

Fixes #547.
Closes #579.
Closes #593.

Related to jquery/api.qunitjs.com#54.
This is the same expression used in the surrounding if-statement which is already known to be true. Isn't this and the next block obsolete?