enh(FilePickerBuilder): Change error thrown when FilePicker is closed

[susnux]

Change error thrown when FilePicker is closed to FilePickerClosed so users can distinguish if error or just closed.

[skjnldsv]

What about returning a cancelable promise?
And check if the promise is cancelled?

[susnux]

That is already done, a promise is returned. But currently a generic Error is used for the rejection.
So you do no know if it is a real error or just aborted by user.

By using reject with a custom error you can check for the error instance to see if it was aborted by the user.

[skjnldsv]

One could easily see it differently 🙈
Cancel with resolve on a null value.
As the dev should check the sanity of the picked value afterwards anyway 🤷

In any case, I think a cancellation would also fit the use case like we do here: https://github.com/nextcloud/server/blob/c9fd5101fceab0d9887b12915435e10492b70674/apps/files/src/services/Files.ts#L80

[susnux]

Cancel with resolve on a null value.

Sounds good but breaking change. Lets do it for 6.x and NC 29 :)