Replace 'instanceof' for IE8 users
Replace 'instanceof' for IE8 users

Summary: `instanceof` doesn't work in IE8 using our weird Error stuff,
just rewrite it to manually check if the error is a ParseError.

Auditors: alpert

sachgits/perseus,learningequality/perseus,ariabuckles/perseus,chirilo/perseus,chirilo/perseus,huangins/perseus,alexristich/perseus,junyiacademy/perseus,ariabuckles/perseus,abe4mvp/perseus,ariabuckles/perseus,sachgits/perseus,iamchenxin/perseus,alexristich/perseus,iamchenxin/perseus,abe4mvp/perseus,ariabuckles/perseus,huangins/perseus,learningequality/perseus,junyiacademy/perseus,daukantas/perseus,daukantas/perseus