Requests to /toddow and /stepwise sometimes result in 500 errors

[ddohler]

When looking at log files to investigate #802 , I noticed numerous server errors resulting from the following traceback; unfortunately I don't know what input caused this issue, and the ToDDoW seemed to be working fine when I visited the site:

ERROR:django.request:Internal Server Error: /api/records/toddow/
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/site-packages/django/core/handlers/exception.py", line 41, in inner
    response = get_response(request)
  File "/usr/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 249, in _legacy_get_response
    response = self._get_response(request)
  File "/usr/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 217, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "/usr/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 215, in _get_response
    response = response.render()
  File "/usr/local/lib/python2.7/site-packages/django/template/response.py", line 107, in render
    self.content = self.rendered_content
  File "/usr/local/lib/python2.7/site-packages/rest_framework/response.py", line 72, in rendered_content
    ret = renderer.render(self.data, accepted_media_type, context)
  File "/usr/local/lib/python2.7/site-packages/rest_framework/renderers.py", line 105, in render
    allow_nan=not self.strict, separators=separators
  File "/usr/local/lib/python2.7/site-packages/rest_framework/utils/json.py", line 28, in dumps
    return json.dumps(*args, **kwargs)
  File "/usr/local/lib/python2.7/json/__init__.py", line 251, in dumps
    sort_keys=sort_keys, **kw).encode(obj)
  File "/usr/local/lib/python2.7/json/encoder.py", line 207, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/usr/local/lib/python2.7/json/encoder.py", line 270, in iterencode
    return _iterencode(o, 0)
  File "/usr/local/lib/python2.7/site-packages/rest_framework/utils/encoders.py", line 49, in default
    return tuple(obj)
  File "/usr/local/lib/python2.7/site-packages/django/db/models/query.py", line 250, in __iter__
    self._fetch_all()
  File "/usr/local/lib/python2.7/site-packages/django/db/models/query.py", line 1121, in _fetch_all
    self._result_cache = list(self._iterable_class(self))
  File "/usr/local/lib/python2.7/site-packages/django/db/models/query.py", line 106, in __iter__
    for row in compiler.results_iter(chunked_fetch=self.chunked_fetch):
  File "/usr/local/lib/python2.7/site-packages/django/db/models/sql/compiler.py", line 841, in results_iter
    results = self.execute_sql(MULTI, chunked_fetch=chunked_fetch)
  File "/usr/local/lib/python2.7/site-packages/django/db/models/sql/compiler.py", line 899, in execute_sql
    raise original_exception
DataError: invalid regular expression: invalid escape \ sequence