Replace the undefined that env uses with a more descriptive error

[Gabriella439]

env passed in an undefined value when generating the list of benchmark names. This is just a request to replace the undefined with a more descriptive error message like:

error "Criterion.env could not determine the list of your benchmarks since they force the environment.  See the documentation of `Criterion.env` for more details"

It doesn't have to be exactly that error, but just something more helpful than "Prelude.undefined"

[bos]

Thanks!

[Gabriella439]

You're welcome!