TaskCancelled fails silently upon a no-update error

[chrisli30]

When an a non-existent taskId, for example undefined, is passed into cancelTask, TaskCancelled currently fails silently, creating confusion on whether the submitted extrinsics executed or not.

It should throw a regular TaskCancelled event indicating a failed cancellation. For example, ProxyExecuted has a good example that when an error happens, it returns the error instead of the actual result.