Fix #13 and #14

`new GlyphSet` will never throw that error, because it doesn't invoke `GlyphSet.prototype.rebuildContents`, if you want to provoke it on creation of a GlyphSet, use for sync `GlyphSet.factory(false, staticIO, glyphSetFaulty)`. However, in this case, the old code was rather fine, I think.

Also, for testing if an error is thrown, use `doh.assertError`, in this case the `doh.assertEqual` will not be executet.