The string formatting code has a test to switch to Unicode when %s

The string formatting code has a test to switch to Unicode when %s
sees a Unicode argument.  Unfortunately this test was also executed
for %r, because %s and %r share almost all of their code.  This meant
that, if u is a unicode object while repr(u) is an 8-bit string
containing ASCII characters, '%r' % u is a *unicode* string containing
only ASCII characters!

Fixed by executing the test only for %s.

Also fixed an error message -- %s argument has non-string str()
doesn't make sense for %r, so the error message now differentiates
between %s and %r.