return 404 instead of 400 when the FHIR resource/endpoint is not recognized/not supported

[CarthageKing]

This PR fixes #539.

This fix should also be compatible with DSTU2...

http://hl7.org/fhir/DSTU2/http.html#2.1.0.10.1
http://hl7.org/fhir/DSTU2/http.html#create

...and DSTU1

http://hl7.org/fhir/DSTU1/http.html#update
http://hl7.org/fhir/DSTU1/http.html#create

[coveralls]

Coverage decreased (-0.003%) to 81.525% when pulling b59c24a on CarthageKing:feature-resource_not_supported-fix into 22f796f on jamesagnew:master.