cue: provide a way of creating an error Value

[rogpeppe]

The cue API extensively uses the capability to return Values holding an error rather than
return (Value, error) pairs. However, there is no way to use the API to create a Value holding
an arbitrary error. This means that it's not easily possible for non-core packages to provide
a similar API or to wrap the cue API while preserving the same style of interface.

We should provide this capability, for example by adding a method like this:

package cue

// Error returns a new Value holding the given error.
func (ctx *Context) Error(err error) Value

Another possibility might be to overload Context.Encode: if its argument implements error, it could
return an error value.