numfmt: use UResult in more functions

This commit replaces generic Results with UResults in some key
functions in numfmt. As a result of this, we can provide different
exit codes for different errors, which resolves ~70 failing test
cases in the GNU numfmt.pl test suite.

tests/by-util/test_numfmt.rs

@@ -568,3 +568,8 @@ fn test_suffix_with_padding() {
         .succeeds()
         .stdout_only("     1000pad 2000 3000\n");
 }
+
+#[test]
+fn test_invalid_number_returns_status_2() {
+    new_ucmd!().pipe_in("hello").fails().code_is(2);
+}