The error messages I'm getting from this are quite indirect - there are
wrapped in quite a few layers. Is there a better way to report errors
inside a build utility?

ice-9/eval.scm:387:11: In procedure eval:
ice-9/eval.scm:387:11: Throw to key `srfi-34' with args `(#<condition &message [message: "substit ute: install.sh: pattern failed to match:  -xf"] 11aa480>)'.
builder for `/gnu/store/wdyzzh7rkg47hfp434w72ly9nay1yva1-mes-boot-0.22.drv' failed with exit code 1

Regards,
Jakub Kądziołka