prolog/swipl: update error format for new version

A recent(?) update to swipl changed the error format from

  Warning: some.pl:2:
              Singleton variables: [Y]

to

  Warning: some.pl:2:
  Warning:    Singleton variables: [Y]

The old error handler doesn't report the correct line numbers and
messages on the old format.

I've chosen to add a function that covers the second case and detect it,
rather than rewrite the current function. This way, both versions should
be able to live together.

---

Example file that demonstrates the issue (some.pl above):
```
% vim: ft=prolog
ii(X, Y) :- X.
```

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