cmd/cue: help $blah does not error for non-existent $blah

[myitcv]

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

$ cue version
cue version v0.0.0-20240304165426-7d4d90de3798

go version go1.22.0
      -buildmode exe
       -compiler gc
  DefaultGODEBUG httplaxcontentlength=1,httpmuxgo121=1,tls10server=1,tlsrsakex=1,tlsunsafeekm=1
     CGO_ENABLED 1
          GOARCH arm64
            GOOS linux
             vcs git
    vcs.revision 7d4d90de3798a00807f50373466322771bd0a10a
        vcs.time 2024-03-04T16:54:26Z
    vcs.modified false

Does this issue reproduce with the latest release?

Yes

What did you do?

! exec cue help mod rubbish
stderr 'unknown command "mod rubbish"'

What did you expect to see?

Passing test (for some variation of the stderr expectation)

What did you see instead?

$ ts repro.txtar
> ! exec cue help mod rubbish
[stdout]


Usage:
  cue mod <cmd> [arguments] [flags]
  cue mod [command]

Available Commands:
  get         add and upgrade module dependencies
  init        initialize new module in current directory
  publish     publish the current module to a registry
  tidy        download and tidy module dependencies

Global Flags:
  -E, --all-errors   print all available errors
  -i, --ignore       proceed in the presence of errors
  -s, --simplify     simplify output
      --strict       report errors for lossy mappings
      --trace        trace computation
  -v, --verbose      print information about progress

Use "cue mod [command] --help" for more information about a command.
FAIL: /tmp/testscript1456235653/repro.txtar/script.txtar:1: unexpected command success

i.e. cue help mod rubbish exists with a 0 exit code for an unknown command mod rubbish

[mvdan]

Similar to #2560, but likely not the same fix as cue cmd needs to load a CUE package to figure out what commands are available.