Setting an XMP tag from a value with incorrect type results in a confusing KeyError being raised

(issue originally reported by Bob Swithers by e-mail)

Observed with pyexiv2 0.3.0, still valid in the latest trunk.
Steps to reproduce, on any image:

    import pyexiv2
    metadata = pyexiv2.ImageMetadata("test/data/smiley1.jpg")
    metadata.read()
    metadata['Xmp.dc.description'] = 'foo'

This fails with the following exception:

/home/osomon/dev/pyexiv2-branches/pyexiv2-0.3/src/pyexiv2/metadata.pyc in __setitem__(self, key, tag_or_value)
    271 return getattr(self, '_set_%s_tag' % family)(key, tag_or_value)
    272 except AttributeError:
--> 273 raise KeyError(key)
    274
    275 def _delete_exif_tag(self, key):

This is confusing to the user, because it seems to induce that the problem is with the key (Xmp.dc.description), which is perfectly valid, whereas the real issue is with the type of the tag (this particular one is of type LangAlt).

A correct assignment would be:

    metadata['Xmp.dc.description'] = {'x-default': 'foo'}

Nevertheless, the exception raised in this case should be a TypeError, not a KeyError.