guix system container and friends fail to report intermediate errors

[Vladilen Kozin <vladilen.kozin@gmail.com>]

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Hello. Is it possible to get better error reporting in the following
example?

$ sudo guix system -K -L /home/vlad/Code/fullmeta-guix/channel container
os.scm

And our os.scm imports some services defined in our channel above. Should
there be an error in one of them, however, the stack trace only talks about
inability to find that service symbol but otherwise fails to report any
errors that may've happened when loading modules from our -L location above.

Case in point. In one of the modules, I habitually used Clojury syntax and
wrote:

(define foo [1 2])

(define foo-service
  (list
     (shepherd-service ... #$foo ...)))

(define foo-service-type ...)

Running the above OS derivation reports:
```
Backtrace:
In guix/store.scm:
... bt here ...
ice-9/boot-9.scm:1685:16: In procedure raise-exception:
error: foo-service-type: unbound variable
```

Backtrace has all modules and files either unknown or gensymed - so not
much use.
It isn't wrong, but e.g. attempting to [1 2] in Guile repl would tell you
"Wrong type to apply: 1" and loading this file would give you the location
... probably :)

Feels like defining shepherd services involves some dark arts and buckets
of time when you can't tell what went wrong where.

Thank you

-- 
Best regards
Vlad Kozin

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