If search term contains no letters, extract the digits for search.

This closes #41385309 & #41385647 in Pivotal.
Do not break search if indexes are not created.

Prevents RuntimeError if indexes in ElasticSearch are missing, which is
unlikely but possible.
Merge pull request #45 from alphagov/goods_code_search_improvements

If search term contains no letters, extract the digits for search.

If required we can add in another story for odd number codes