Clarify promise rejection value

[LinusU]

I'm a bit unsure of what to expect back from the async clipboard API when there is a failure. The explainer.adoc here doesn't use the rejection value at all, and the spec simply says "reject |p|".

clipboard-apis/index.bs

Line 676 in 2ac3c0b

1. If |r| is not "granted", then reject |p|

Is the intention that the Promise should simply reject with the value undefined instead of an Error instance?

This is how we have interpreted it here: feross/clipboard-copy#24

I feel that this goes against best practices, and I think there is a ton of code in the wild that does something similar to:

runAllMythings().catch((err) => {
  console.log(err.stack)
})

Wouldn't it be nice to reject with an error, maybe reject(new Error('Persmission for writing to clipboard was not granted'))?

[gked]

@LinusU clipboard API should probably be able to relay PermissionState value back to the developer. @garykac what do you think if we added new property to DataTransfer interface? Something like readonly attribute PermissionState state; with some default value.

[garykac]

We should definutely return better DOMExceptions, but I don't think we want to touch DataTransfer.

E.g., the spec should be updated to read:

   1. If |r| is not granted, then reject |p| with a "NotAllowedError" DOMException.

While the spec currently uses a DataTransfer object for read/write, I believe that we'll need to change that to a more appropriate object because of recent issues that were raised. Specifically #41 and #44 and this comment from the TAG review.

[gked]

@garykac NotAllowedError seems a bit ambiguous. But I guess for the lack of a better alternative, I think, this should work. I am checking with Travis about why DataTransfer was questioned during TAG review.

[garykac]

Re: "NotAllowedError", see the comments here for the recommendation to use this for denied requests.

Questions about the appropriateness of DataTransfer came about before the TAG review because it doesn't quite match what we'll need to do for the clipboard:

• Images can be on the clipboard multiple times (in different formats)
• We'll need placeholders for delay-generated data

We could hack these things on top of DataTransfer, but that's not necessarily the best approach.