Excluding a field using Meta.exclude in a base class

Since #826, because the check also applies on Meta parameters, this fails because there is no `"bar"` field in the schema:

```py
class MySchema(Schema):
foo = fields.Field()

class Meta:
exclude = ('bar', )

MySchema()
```

In my app, I would like to exclude a field from all schemas, so I'd like to add `exclude=field_name` to my base schema class. But since the field is not in all schemas, it fails on the ones that don't contain the field.

I do this as a workaround, but it sucks:

```py
class BaseSchema(Schema):
def _init_fields(self):
super()._init_fields()
self.fields.pop('field_name', None)
self.load_fields.pop('field_name', None)
self.dump_fields.pop('field_name', None)
```

Any ideas how to do this correctly?

Maybe this check is too restrictive after all.

Start by tracing Schema._init_fields and the handling of Meta.exclude, using the example with an absent "bar" field to reproduce the failure. Compare that behavior with the BaseSchema workaround; done means a base-class exclusion can be inherited by schemas that do not define the excluded field without raising an error.

Written by the indexing model from the issue text.