Improve handling of non-JSON responses

[bhornseth]

Occassionally, the API (or more likely: the load balancer) will return anon-JSON response and we're handling that poorly:

MultiJson::ParseError lexical error: invalid char in json text.
                                       <h1>This website is under heavy
                     (right here) ------^
  
  /home/httpd/mls_search/prod/shared/bundle/ruby/2.3.0/gems/multi_json-1.12.1/lib/multi_json/adapters/yajl.rb:11:in `parse'
  /home/httpd/mls_search/prod/shared/bundle/ruby/2.3.0/gems/multi_json-1.12.1/lib/multi_json/adapters/yajl.rb:11:in `load'
  /home/httpd/mls_search/prod/shared/bundle/ruby/2.3.0/gems/multi_json-1.12.1/lib/multi_json/adapter.rb:21:in `load'
  /home/httpd/mls_search/prod/shared/bundle/ruby/2.3.0/gems/multi_json-1.12.1/lib/multi_json.rb:122:in `load'
  /home/httpd/mls_search/prod/shared/bundle/ruby/2.3.0/gems/spark_api-1.4.13/lib/spark_api/faraday_middleware.rb:20:in `on_complete'