Provide reasonable error when first argument to EnumField is not an Enum

[MattF-NSIDC]

Hello! I made a stupid mistake refactoring and didn't realize it. The error message didn't help :)

Modified example from front page:

from marshmallow import Schema
from enum import Enum
from marshmallow_enum import EnumField


class Something:
   foo = 1

class StopLight(Enum):
    green = 1
    yellow = 2
    red = 3

class TrafficStop(Schema):
    light_color = EnumField(Something)

schema = TrafficStop()

print(schema.dump({'light_color': StopLight.red}).data)
print(schema.load({'light_color': 'red'}).data)

Expected failure on schema instantiation:

RuntimeError: Received "Something" which is not of type Enum

Actually got failure on schema.load call:

{'light_color': 'red'}
Traceback (most recent call last):
  File "/opt/miniconda/envs/hermes-api/lib/python3.7/site-packages/marshmallow_enum/__init__.py", line 95, in _deserialize_by_name
    return getattr(self.enum, value)
AttributeError: type object 'Something' has no attribute 'red'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "test.py", line 20, in <module>
    print(schema.load({'light_color': 'red'}).data)
  File "/opt/miniconda/envs/hermes-api/lib/python3.7/site-packages/marshmallow/schema.py", line 580, in load
    result, errors = self._do_load(data, many, partial=partial, postprocess=True)
  File "/opt/miniconda/envs/hermes-api/lib/python3.7/site-packages/marshmallow/schema.py", line 660, in _do_load
    index_errors=self.opts.index_errors,
  File "/opt/miniconda/envs/hermes-api/lib/python3.7/site-packages/marshmallow/marshalling.py", line 295, in deserialize
    index=(index if index_errors else None)
  File "/opt/miniconda/envs/hermes-api/lib/python3.7/site-packages/marshmallow/marshalling.py", line 68, in call_and_store
    value = getter_func(data)
  File "/opt/miniconda/envs/hermes-api/lib/python3.7/site-packages/marshmallow/marshalling.py", line 288, in <lambda>
    data
  File "/opt/miniconda/envs/hermes-api/lib/python3.7/site-packages/marshmallow/fields.py", line 265, in deserialize
    output = self._deserialize(value, attr, data)
  File "/opt/miniconda/envs/hermes-api/lib/python3.7/site-packages/marshmallow_enum/__init__.py", line 82, in _deserialize
    return self._deserialize_by_name(value, attr, data)
  File "/opt/miniconda/envs/hermes-api/lib/python3.7/site-packages/marshmallow_enum/__init__.py", line 97, in _deserialize_by_name
    self.fail('by_name', input=value, name=value)
  File "/opt/miniconda/envs/hermes-api/lib/python3.7/site-packages/marshmallow_enum/__init__.py", line 100, in fail
    kwargs['values'] = ', '.join([str(mem.value) for mem in self.enum])
TypeError: 'type' object is not iterable

[justanr]

Would definitely review a PR for this