fix(spell): fail gracefully on missing ispell-program-name

Instead of stopping things dead with a hard error, emit a more readable
warning, instead.

modules/checkers/spell/config.el

@@ -62,7 +62,9 @@
 
     (_ (doom-log "Spell checker not found. Either install `aspell', `hunspell' or `enchant'")))
 
-  (ispell-check-version))
+  (if (executable-find ispell-program-name)
+      (ispell-check-version)
+    (warn "Can't find %s in your $PATH" ispell-program-name)))
 
 
 ;;