[w3c/manifest] Processing the purpose member should return the same value in both error conditions (#513)

Maybe that was on purpose (no pun intended) but it sounds fairly odd that `purpose` parsing will return `undefined` when the value isn't a string but a set with `"any"` if the value is a string but doesn't contain a known value.

I would suggest to do something similar to `icons` where the same value (empty array) is returned regardless of the type of errors. Here, we could return an empty set.

(As a side note, is space separated keywords more common in JSON than comma separated? HTML uses comma separated most of the time, doesn't it?)

CC @marcoscaceres 

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