Opaque error message when "length":"30" is used

[jbfaden]

I wonder if this error could be made less opaque:

http://localhost:8080/HapiServer/hapi/info?id=solar_wind_mag_pt3m&parameters=bt_min
  ✗ is.HAPIJSON(): Expect body to be valid HAPI 3.1 'info' schema.
  Got:   [
           "parameters[0].length is not exactly one from [subschema 0],</Ref>"
       ]

The error was that "length":"30" was used when "length":30 should have been. I realize that this is the schema validator output, but we should either check that in the semmantic test or write the schema so that a better error is presented.

[rweigel]

Closing and adding to hapi-server/data-specification-schema#17