Make Package._metadata_exception() use unicode format strings in order

Make Package._metadata_exception() use unicode format strings in order
to avoid UnicodeDecodeError in python-2.x.

This issue was reported in forum thread
http://forums.gentoo.org/viewtopic-t-844623.html where the following
traceback was posted:

  File "/usr/lib/portage/pym/_emerge/FakeVartree.py", line 195, in _pkg
    type_name="installed")
  File "/usr/lib/portage/pym/_emerge/Package.py", line 64, in __init__
    self._validate_deps()
  File "/usr/lib/portage/pym/_emerge/Package.py", line 93, in _validate_deps
    self._metadata_exception(k, e)
  File "/usr/lib/portage/pym/_emerge/Package.py", line 242, in _metadata_exception
    "%s: %s in '%s'" % (k, e, path))
UnicodeDecodeError: 'ascii' codec can't decode byte 0xef in position 22: ordinal not in range(128)