beep: show error upon failure to open sound device

If beep cannot open /dev/dsp provide more information to aid the user
in diagnosing a problem.

Reviewed by: christos
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 83325e7b738ce87d88553585b135b0e7d15997a6)
(cherry picked from commit 7da246dc4ffeba38ad723a1cb72a19511292c840)