Handle encoding in case of error

When an invalid query with non-ascii characters is sent to the server, a UnicodeDecodeError is raised while parsing the error message.

Test script to reproduce with the Solr/jetty example:

```python
# -*- coding: utf-8 -*-
from pysolr import Solr

conn = Solr('http://127.0.0.1:8983/solr/')
s = u'äüö "'
conn.search(s)
```