feat(nix/yants): improve error message for errornous predicate r/3463

while trying to yantsify `mkSecrets` in https://cl.tvl.fyi/c/depot/+/4688,
I(zseri) needed to debug a failing evaluation which boiled down
to a result.ok containing something which wasn't boolean,
but the error message didn't indicate where that value came from.

I debugged yants and found that the only place which didn't
simply combine boolean values or use functions which always
return booleans, I managed to isolate the error to the
`pred v` expression. To avoid the necessity to debug yants
to find this, I improve the error message for this case
to mention that
- a restriction predicate is invalid
- what's the name of the failing restriction
- the unexpected predicate return value

Change-Id: I6c570a33ccc5afc445f208e2e8855c49fb37abaf
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4698
Tested-by: BuildkiteCI
Reviewed-by: zseri <zseri.devel@ytrizja.de>
Reviewed-by: tazjin <mail@tazj.in>
Autosubmit: zseri <zseri.devel@ytrizja.de>

0 files changed, 0 insertions, 0 deletions