feat: modify return codes

Modify exit codes. `cheat` now returns an exit code value `2` on errors
pertaining to a cheatsheet not being found.

BREAKING CHANGE

doc/cheat.1

@@ -203,6 +203,13 @@ If set, autocompletion scripts will attempt to integrate with
 \f[B]fzf\f[].
 .RS
 .RE
+.SH RETURN VALUES
+.IP "0." 3
+Successful termination
+.IP "1." 3
+Application error
+.IP "2." 3
+Cheatsheet(s) not found
 .SH BUGS
 .PP
 See GitHub issues: <https://github.com/cheat/cheat/issues>

doc/cheat.1.md

@@ -163,6 +163,15 @@ set, all other config paths will be ignored.
 
 : If set, autocompletion scripts will attempt to integrate with **fzf**.
 
+RETURN VALUES
+=============
+
+0. Successful termination
+
+1. Application error
+
+2. Cheatsheet(s) not found
+
 
 BUGS
 ====