Indicate which variables were involved during a joined failure

[tel]

Right now, the runtime evaluator returns an errors type as a list specifying either a missing value or a joined error, but those joined errors occur in specific branches of the applicative so we ought to be able to detect where and capture the error better.

DATBASE_URL: missing
BAZ_QUUX: missing
FOO_BAR: some joined error

instead of

Missing: DATABASE_URL, BAZ_QUUX
some joined error