Uninformative error message when GPG is not installed

[xandkar]

$ ./lib/main/opass add
Password/note (P/n): P
Name: Dude Guy
Location: Some Place
Username: dude
Password: y#yi,%vUqe3Lda^4o%1fGD*5
Add, retry, cancel? (A/r/c): A
Uncaught exception:

  (lib/shell.ml.Process.Failed
   ((command
     ((program /bin/sh) (arguments (-c "gpg -a --symmetric 2>/dev/null"))))
    (status (Exited 127)) (stdout "") (stderr "")))