Introduce invariant that _invalid_pofile() takes unicode line.

Introduce invariant that _invalid_pofile() takes unicode line.

This makes debugging and reasoning about the code easier;
otherwise it is surprising that sometimes `line` is a unicode
and sometimes not.

So far, when it was not, it could either be only `""` or
`'Algo esta mal'`; thus this commit makes those two u"" strings.
In all other cases, it was guaranteed that it's unicode,
because all code paths leading to `_invalid_pofile()` went through

    if not isinstance(line, text_type):
        line = line.decode(self.catalog.charset)

before.