A common error with benchmarking is to call the dir bench rather than benches.

This is the error message a user currently gets:

error: failed to parse manifest at `<snip>/Cargo.toml`

Caused by:
  can't find `my_benchmark` bench, specify bench.path

It fails the esteban test as it doesn't hint as to what we've done wrong.

At a minimum can we change it to this please?

Caused by:
  can't find `benches/my_benchmark` bench, specify bench.path

That way they stand a chance of spotting there's an issue. Personally I think it's worth testing for a bench dir and having an explicit message because so many people must do this and then kick themselves when the penny drops.