ckanext.scheming.errors.SchemingException: validator/converter not found: u'scheming_multiple_choice_output'

This might be related in some ways to #199.

I have a CKAN installation, coupled with Wordpress, on top of which is activated the ckanext-scheming plugin. I have a custom scheming json which is used to enrich the resources fields.

While trying to perform a search index rebuild, I get the following errors :
```python-traceback
Indexing dataset 1/1113Traceback (most recent call last):
File "/usr/lib/ckan/default/bin/paster", line 11, in
sys.exit(run())
File "/usr/lib/ckan/default/local/lib/python2.7/site-packages/paste/script/command.py", line 102, in run
invoke(command, command_name, options, args[1:])
File "/usr/lib/ckan/default/local/lib/python2.7/site-packages/paste/script/command.py", line 141, in invoke
exit_code = runner.run(args)
File "/usr/lib/ckan/default/local/lib/python2.7/site-packages/paste/script/command.py", line 236, in run
result = self.command()
File "/usr/lib/ckan/default/src/ckan/ckan/lib/cli.py", line 579, in command
self.rebuild()
File "/usr/lib/ckan/default/src/ckan/ckan/lib/cli.py", line 602, in rebuild
quiet=self.options.quiet)
File "/usr/lib/ckan/default/src/ckan/ckan/lib/search/__init__.py", line 194, in rebuild
defer_commit
File "/usr/lib/ckan/default/src/ckan/ckan/lib/search/index.py", line 102, in update_dict
self.index_package(pkg_dict, defer_commit)
File "/usr/lib/ckan/default/src/ckan/ckan/lib/search/index.py", line 122, in index_package
pkg_dict, schema, 'package_show')
File "/usr/lib/ckan/default/src/ckan/ckan/lib/plugins.py", line 268, in plugin_validate
result = plugin.validate(context, data_dict, schema, action)
File "/usr/lib/ckan/default/src/ckanext-scheming/ckanext/scheming/plugins.py", line 268, in validate
f, scheming_schema, False)
File "/usr/lib/ckan/default/src/ckanext-scheming/ckanext/scheming/plugins.py", line 417, in _field_output_validators
f['output_validators'], f, schema)
File "/usr/lib/ckan/default/src/ckanext-scheming/ckanext/scheming/validation.py", line 301, in validators_from_string
v = get_validator_or_converter(p)
File "/usr/lib/ckan/default/src/ckanext-scheming/ckanext/scheming/validation.py", line 319, in get_validator_or_converter
raise SchemingException('validator/converter not found: %r' % name)
ckanext.scheming.errors.SchemingException: validator/converter not found: u'scheming_multiple_choice_output'
```
What am I missing? Where should I look for the validator/converted mentionned above? Is it a specific file which is expected somewhere in the system?
[wonderful_metadata_scheming.txt](https://github.com/ckan/ckanext-scheming/files/2644968/wonderful_metadata_scheming.txt)
#199

Start with the attached wonderful_metadata_scheming.txt and inspect the output_validators entry referenced by the traceback. Read ckanext/scheming/validation.py at validators_from_string and get_validator_or_converter, then follow the rebuild path through ckan/lib/search/index.py. Reproduce the search index rebuild and determine whether the named validator is missing or incorrectly configured.

Written by the indexing model from the issue text.