eval: improve error message eagerness around required fields

[myitcv]

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

$ cue version
cue version v0.13.0-alpha.1.0.20250322080417-cfbeb48088c9

go version go1.24.0
      -buildmode exe
       -compiler gc
  DefaultGODEBUG gotestjsonbuildtext=1,multipathtcp=0,randseednop=0,rsa1024min=0,tlsmlkem=0,x509rsacrt=0,x509usepolicies=0
     CGO_ENABLED 1
          GOARCH arm64
            GOOS linux
         GOARM64 v8.0
             vcs git
    vcs.revision cfbeb48088c9674480496969ad85a93602e9778c
        vcs.time 2025-03-22T08:04:17Z
    vcs.modified false
cue.lang.version v0.13.0

Does this issue reproduce with the latest release?

Yes

What did you do?

# use evalv3 - soon to be the default
env CUE_EXPERIMENT=evalv3=1

# cars
! exec cue vet -c cars.yaml x.cue
stderr 'cars\.0\.extra: field not allowed'
stderr 'cars\.0\.year: field is required but not present'

-- cars.yaml --
cars:
  - type: "car"
    name: "Toyota"
    extra: "extra"
-- x.cue --
#Car: {
	type!: "car"
	name!: string
	year!: string
}

cars?: [...#Car]

What did you expect to see?

Passing test... perhaps.

What did you see instead?

Note the use of testscript -continue:

# use evalv3 - soon to be the default (0.000s)
# cars (0.026s)
> ! exec cue vet -c cars.yaml x.cue
[stderr]
cars.0.extra: field not allowed:
    ./cars.yaml:4:5
[exit status 1]
> stderr 'cars\.0\.extra: field not allowed'
> stderr 'cars\.0\.year: field is required but not present'
FAIL: repro.txtar:7: no match for `cars\.0\.year: field is required but not present` found in stderr

In this example we don't see the error message about this missing year field not being present. Is it possible, given that we are running in vet -c mode, to eagerly show this message? Yes, it's conceivable that other fields not being in error could lead to the field being contributed (i.e. via a comprehension) but it seems more useful to show the error message given the error state.