Don't throw unhandled exception when EOCDR is not found

On Chrome I get a ```Uncaught Error: IndexSizeError: DOM Exception 1``` when attempting to read a file with random data as a zip file. This error happens in the `onload` callback of FileReader so it remains uncaught. This pull requests improves the error handling.

Possibly related to https://github.com/gildas-lormeau/zip.js/issues/74