Include path in error message when get_metadata() raises e.g. UnicodeDecodeError

[cjerdonek]

This issue is another manifestation of issue #1664 ("Include location of bad METADATA files in errors"), but for the value.decode('utf-8') line in NullProvider.get_metadata():

setuptools/pkg_resources/__init__.py

Lines 1416 to 1420 in 8aeff6b

	def get_metadata(self, name):
	if not self.egg_info:
	return ""
	value = self._get(self._fn(self.egg_info, name))
	return value.decode('utf-8') if six.PY3 else value

This came up recently in this pip issue (and probably other pip issues -- this is just one example): pypa/pip#6566

Rather than re-opening #1664, it probably makes sense to have a new issue. It's unknown to me how many code paths in pkg_resources affect the larger issue. PR #1706 addressed the version-is-missing case.

[cjerdonek]

I posted a fix for this a week ago here: #1791