Files with broken directory file entries are not handled gracefully

[dottedmag]

If a file with a broken CHM directory file entry is passed to Archmage, it prints a traceback:

UnicodeDecodeError: 'utf-8' codec can't decode byte 0x88 in position 3: invalid start byte

These malformed files should be detected and ignored, and a warning presented to the user to notify that some links in the output will be broken.