MarshalStream eats error message details

[grihabor]

Is your feature request related to a problem? Please describe.
Here is an example:

import (
	"encoding/yaml"
)

config: {
	a: int
	b: string
	c: 42
}

output: yaml.MarshalStream([config])

Now if I run

cue export --expression output ./main.cue 

it says

output: error in call to encoding/yaml.MarshalStream: incomplete value _:
    ./main.cue:11:9

which doesn't tell me anything about which fields are incomplete.

Meanwhile, if I run

cue export --expression config ./main.cue

it shows exactly what's wrong

config.a: incomplete value int:
    ./main.cue:6:5
config.b: incomplete value string:
    ./main.cue:7:5

Describe the solution you'd like
In my example, I want the output for the command

cue export --expression output ./main.cue

be something like

output.0.config.a: incomplete value int:
    ./main.cue:6:5
output.0.config.b: incomplete value string:
    ./main.cue:7:5

[mxey]

I think this is a duplicate of #2718