Incomplete errors should not unify with values.
-- in.cue --
a: or([]) @test(err, code=incomplete, contains="empty list", pos=[0:4])
a: "t"

b: _|_ @test(err, code=user, contains="explicit error", pos=[0:4])
b: "t"
-- out/errors.txt --
[incomplete] a: error in call to or: empty list in call to or:
    ./in.cue:1:4
[user] explicit error (_|_ literal) in source:
    ./in.cue:4:4
-- out/eval/stats --
Leaks:  0
Freed:  4
Reused: 0
Allocs: 4
Retain: 0

Unifications: 4
Conjuncts:    6
Disjuncts:    0
