SystemError if assign_cb raises Exception

Description
===========
A `SystemError` exception is raised when calling `poll` or `close` on a consumer if the `on_assign` callback raises an Exception.
```python
def log(self, level, msg, *args, **kwargs):
"""
Log 'msg % args' with the integer severity 'level'.

To pass exception information, use the keyword argument exc_info with
a true value, e.g.

logger.log(level, "We have a %s", "mysterious problem", exc_info=1)
"""
> if not isinstance(level, int):
E SystemError: returned a result with an error set
```

I think this is due to the cpython code not clearing the python exception during callbacks and subsequent callbacks executing python code.
Reading the docs, I get the impression that the librdkafka callbacks which execute python code should clear exceptions `PyErr_Clear` and restore them when python code is calling cpython functions (e.g. check and set the error when `consumer.poll` is called). If this is right, we could probably replace the `crashed` counter with the exception info.

How to reproduce
================
```python
import logging
logging.basicConfig(level=logging.DEBUG)
from confluent_kafka import Producer
from confluent_kafka import Consumer
prod = Producer(
{
'bootstrap.servers': servers,
'debug': 'all'
},
logger=logging.getLogger('')
)
prod.produce('test', b'data')
assert prod.flush(10) == 0
cons = Consumer(
{
'bootstrap.servers': servers,
'group.id': 'test',
'auto.offset.reset': 'smallest',
'debug': 'all'
},
logger=logging.getLogger('')
)

def _on_assign(self, *args, **kwargs):
raise Exception("Oops")

cons.subscribe(['test'], on_assign=_on_assign, on_revoke=_on_assign)
msg = cons.poll(10)
assert msg is not None
cons.close()
```

I can provide the debug logs if necessary.

Checklist
=========
Please provide the following information:

- [x] `confluent_kafka.version(), confluent_kafka.libversion() == (('1.2.2', 16908800), ('1.2.2', 16909055))`
- [x] 0.10.2.1
- [ ] Client configuration: `{...}`
- [ ] Operating system:
- [ ] Provide client logs (with `'debug': '..'` as necessary)
- [ ] Provide broker log excerpts
- [ ] Critical issue