It's possible that gpg will work fine without $GPG_TTY being set (e.g., when
using a GUI pinentry program). Therefore, the message should be classified as
a warning, not an error.
Also, use a single :echom to ensure the user sees the entire message, not just
"gpg-agent might not work."
First part of GH #1
Signed-off-by: James McCoy <vega.james@gmail.com>