Don't raise exception on invalid UTF-8 output

This could be reproduced with:

    ./test/spec_tests.py --program kramdown --pattern Enti

with Kramdown version 1.5.0.

The main problem is that it would stop future tests from being carried out.

After this commit it is just counted as a failure.

The error message printed is of the form:

    Unicode error: 'utf8' codec can't decode byte 0xfc in position 8: invalid start byte
    '<p># \xd3\x92 \xcf\xa0 \xef\xbf\xbd</p>\n'
    '<p># \xd3\x92 \xcf\xa0 \xfc\x85\xb8\xb0\xa9\xb8</p>\n'