make errors' Imp_def a term representing a context

[ichiban]

?- f(X).
2022/05/14 15:23:19 error(existence_error(procedure, f/1), root)
?- call(f(X)).
2022/05/14 15:23:23 error(existence_error(procedure, f/1), call/1)

[codecov]

Codecov Report

Merging #225 (42bf1e7) into main (f788ec4) will increase coverage by 0.02%.
The diff coverage is 99.10%.

@@            Coverage Diff             @@
##             main     #225      +/-   ##
==========================================
+ Coverage   96.90%   96.92%   +0.02%     
==========================================
  Files          21       21              
  Lines        6301     6319      +18     
==========================================
+ Hits         6106     6125      +19     
+ Misses        161      160       -1     
  Partials       34       34              

Impacted Files	Coverage Δ
engine/builtin.go	96.25% <97.95%> (-0.05%)	⬇️
engine/clause.go	97.82% <100.00%> (ø)
engine/env.go	98.17% <100.00%> (+1.39%)	⬆️
engine/exception.go	100.00% <100.00%> (ø)
engine/iterator.go	100.00% <100.00%> (ø)
engine/number.go	100.00% <100.00%> (ø)
engine/promise.go	100.00% <100.00%> (ø)
engine/stream.go	100.00% <100.00%> (ø)
engine/vm.go	82.74% <100.00%> (ø)
interpreter.go	94.88% <100.00%> (ø)
... and 2 more

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