cue cmd: improve error message for non-existing commands

[edwintorok]

What version of CUE are you using (cue version)?

$ cue version
v0.2.2

(also reproduced on latest master version)

Does this issue reproduce with the latest release?

Yes, tested v0.2.2 and v0.3.0-alpha1.0.20200730155834-304b02e40842, different error message though.

What did you do?

cat >prompter_tool.cue <<EOF
package foo

import (
        "tool/cli"
        "tool/exec"
        "tool/file"
)

city: "Amsterdam"

// Say hello!
command: prompter: {
        // save transcript to this file
        var: file: *"out.txt" | string @tag(file)

        ask: cli.Ask & {
                prompt:   "What is your name?"
                response: string
        }

        // starts after ask
        echo: exec.Run & {
                cmd:    ["echo", "Hello", ask.response + "!"]
                stdout: string // capture stdout
        }

        // starts after echo
        file.Append & {
                filename: var.file
                contents: echo.stdout
        }

        // also starts after echo
        print: cli.Print & {
                text: echo.stdout
        }
}
EOF

What did you expect to see?

Example from go help cmd working (1st one does, 2nd one doesn't).
A prompt and then a message on stdout.

What did you see instead?

v0.2.2 no prompt, and:

go/bin/cue cmd -v prompter
command.prompter.contents: invalid bytes argument for field "contents": non-concrete value string:
    ./prompter_tool.cue:12:10

v0.3.0-alpha1.0.20200730155834-304b02e40842 no prompt, and:

go/bin/cue cmd -v prompter
command.prompter.contents: invalid bytes argument for field "contents": incomplete cause disjunction:
    ./prompter_tool.cue:12:10

[chai2010]

see #467 #468

[mpvl]

This should be fixed with Ask now being supported. Obviously, there is another problem here in that it should have reported that as an error, and not the obscure errors it returned instead. So keeping this open for now.

[cueckoo]

This issue has been migrated to cue-lang/cue#479.

For more details about CUE's migration to a new home, please see cue-lang/cue#1078.