add a file error_log.pdf which produces an error when text is read or
an image is rendered
add a fixture document_with_error() opening that file
add a test test_document_with_error() which captures the stderr on the
file descriptor and tests that the error message is produced only if
the last invocation was not False
Follow up for https://github.com/cbrunet/python-poppler/pull/61, issue https://github.com/cbrunet/python-poppler/issues/49.