[FileAPI] Stop using DOMError (#11)

DOMError is gone from https://dom.spec.whatwg.org/ and we're looking to get rid of it in Blink:
https://code.google.com/p/chromium/issues/detail?id=460725

CC @annevk (DOM) and @inexorabletash (IndexedDB has a similar situation)

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/FileAPI/issues/11