Misleading exception raised when metadata not read

[Olivier Tilloy]

In the 0.1.x series, when trying to access the metadata of an image that had not been read() yet, an IOError with a relevant error message was raised.
The same behaviour is expected in the 0.2.x series, but instead an AttributeError with a misleading error message is raised.

Script to reproduce:

    import pyexiv2
    m = pyexiv2.ImageMetadata("test/data/smiley1.jpg")
    m.exif_keys

Expected result: IOError: Image metadata has not been read yet
Current result: AttributeError: 'NoneType' object has no attribute '_exifKeys'

[Olivier Tilloy (osomon)]

Fixed with revision 337 of the trunk.