Replace ChannelStoragePropError with RejectedProp

[tudddorrr]

Type consolidation

• Removed the now-redundant ChannelStoragePropError class (which had identical fields key, error, message as RejectedProp)
• Replaced all ChannelStoragePropError[] references with the pre-existing RejectedProp[] in ChannelsAPI.OnChannelStoragePropsFailedToSet, the response DTO, and sample UI code
• Unifies failed-prop error reporting across channels so callers see a single type (RejectedProp) whether props are rejected during a set operation or via the existing OnChannelPropsRejected event