Clean up the output when an unknow type is found

Makes output go from:

```
Traceback (most recent call last):
  File "foo.py", line 49, in <module>
    'id': None,
  File "/home/sontek/.virtualenvs/jsonschema/jsonschema/jsonschema/validators.py", line 116, in validate
    for error in self.iter_errors(*args, **kwargs):
  File "/home/sontek/.virtualenvs/jsonschema/jsonschema/jsonschema/validators.py", line 95, in iter_errors
    for error in errors:
  File "/home/sontek/.virtualenvs/jsonschema/jsonschema/jsonschema/_validators.py", line 219, in type_draft3
    if validator.is_type(instance, type):
  File "/home/sontek/.virtualenvs/jsonschema/jsonschema/jsonschema/validators.py", line 121, in is_type
    raise UnknownType(type)
jsonschema.exceptions.UnknownType: unknown

```


to:


```
Traceback (most recent call last):
  File "foo.py", line 49, in <module>
    'id': None,
  File "/home/sontek/.virtualenvs/jsonschema/jsonschema/jsonschema/validators.py", line 116, in validate
    for error in self.iter_errors(*args, **kwargs):
  File "/home/sontek/.virtualenvs/jsonschema/jsonschema/jsonschema/validators.py", line 95, in iter_errors
    for error in errors:
  File "/home/sontek/.virtualenvs/jsonschema/jsonschema/jsonschema/_validators.py", line 219, in type_draft3
    if validator.is_type(instance, type):
  File "/home/sontek/.virtualenvs/jsonschema/jsonschema/jsonschema/validators.py", line 121, in is_type
    raise UnknownType(type, instance, self.schema)
jsonschema.exceptions.UnknownType: 
unknown type unknown, in schema:
    {'additionalProperties': False, 'type': 'unknown', 'properties': {'id': {'pattern': '^\\d+$', 'type': [None, 'string']}}}

On instance:
    {'id': None}

```