invalid zip code should report error in standard json response

[dnesting]

We've seen a few dozen of these errors in production for /v1/schools/ over the last few hours:

NoMethodError: undefined method `[]' for nil:NilClass
/home/vcap/app/lib/data_magic/query_builder.rb: 132:in `search_location'
/home/vcap/app/lib/data_magic/query_builder.rb:  30:in `generate_squery'
/home/vcap/app/lib/data_magic/query_builder.rb:  13:in `from_params'
/home/vcap/app/lib/data_magic.rb:  69:in `search'

[dnesting]

This appears to happen when you search with an invalid ZIP code and non-empty radius, e.g.:

https://collegescorecard.ed.gov/search/?zip=00002&distance=5&sort=advantage:desc

Error
<h1>Internal Server Error</h1>

[dnesting]

Since this can not match any results, we should either generate an error to that effect, or perhaps simply validate on query_builder.rb:127 that location is defined.

[ultrasaurus]

To reproduce using the default sample-data, this query shows the same failure: http://localhost:3000/v1/cities?zip=00002&distance=5

Notice when the zip is valid, we get a json response: http://localhost:3000/v1/cities?zip=94132&distance=5

I think we need to address some missing logic at a few levels:

1. the ZipCode library should have a well-defined behavior for invalid zip codes -- need to add an example to zipcode_spec.rb for invalid zip code -- raising an error with the message "invalid zip code" would be a way to help future developers. For our case, I think adding a valid? method would make sense
2. the we could add a test to error_checker_spec.rb a check for this, and of course the corresponding implementation

[photomattmills]

I think this does the trick (mostly done at the hacknight, but tidied up just now): #240