Install script: fix error message if completion directory does not exist

bin/install

@@ -12,11 +12,16 @@ readonly executable_path=$local_bin/$executable_name
 stack install
 
 echo >&2
-if [[ -n "${LOCAL_ZSH_COMPLETION_PATH+set}" && -d "$LOCAL_ZSH_COMPLETION_PATH" ]]
+if [[ -n "${LOCAL_ZSH_COMPLETION_PATH+set}" ]]
 then
-	ZSH_COMPLETION_SCRIPT_PATH=$LOCAL_ZSH_COMPLETION_PATH/_$executable_name
-	$executable_path --zsh-completion-script $executable_path > $ZSH_COMPLETION_SCRIPT_PATH
-	echo "Installed zsh completions for $executable_name to $ZSH_COMPLETION_SCRIPT_PATH" >&2
+	if [[ -d "$LOCAL_ZSH_COMPLETION_PATH" ]]
+	then
+		ZSH_COMPLETION_SCRIPT_PATH=$LOCAL_ZSH_COMPLETION_PATH/_$executable_name
+		$executable_path --zsh-completion-script $executable_path > $ZSH_COMPLETION_SCRIPT_PATH
+		echo "Installed zsh completions for $executable_name to $ZSH_COMPLETION_SCRIPT_PATH" >&2
+  else
+		echo "Not installing completions, LOCAL_ZSH_COMPLETION_PATH is set but isn't a directory ($LOCAL_ZSH_COMPLETION_PATH)." >&2
+	fi
 else
   echo "Not installing completions, set LOCAL_ZSH_COMPLETION_PATH to install them." >&2
 fi