builtins: use a warning for declare -p with undefined variables

Instead of raising an error. The exit status of the command should be
enough to signify failure.

This avoids the issue where `declare -p UNDEFINED_VAR` is being used to
determine if a variable is defined causing errors when sourcing the
related file or string.

Author: radhermit

builtins/declare.def

@@ -398,7 +398,7 @@ declare_internal (WORD_LIST *list, int local_var)
 	    pflag = show_name_attributes (list->word->word, nodefs);
 	  if (pflag)
 	    {
-	      sh_notfound (list->word->word);
+	      builtin_warning (_("%s: not found"), list->word->word);
 	      any_failed++;
 	    }
 	}