Improve KMIP error code for invalid key spec

When using KMIP plugin for backend, Generate RSA using the orders interface with passphrase, returns 500 status. This feature is not yet supported for KMIP backend. So in this case a 4XX Error code [Bad Request] would be an appropriate error code instead of 500.

Detailed description:

1) Version of Barbican we are using:

   - stable/master

2) Reproduce steps:
    a) Install via devstack, with following barbican post-configuration in local.conf:

       #...
        [[post-config|$BARBICAN_CONF]]
        [secretstore]
        namespace = barbican.secretstore.plugin
        enabled_secretstore_plugins = kmip_plugin

       [kmip_plugin]
       username = 'yourUserName'
       password = '*******'
       host = 11.222.333.44
       port = 6666
       keyfile = '/opt/stack/Certs/user_unencrypted.key'
       certfile = '/opt/stack/Certs/user.crt'
       ca_certs = '/opt/stack/Certs/LocalCA.crt'
      #...

   b) Generate RSA using the orders interface:
        curl -X POST http://localhost:9311/v1/orders -H 'Accept: application/json' -H 'Content-Type: application/json' -H 'X-Project-Id:12345' -d '{"meta":{"bit_length": 2048, "passphrase": "password", "name": "ordered rsacontainer1", "algorithm": "rsa", "mode": "cbc"}, "type": "asymmetric"}'

       {"order_ref": "http://localhost:9311/v1/orders/7bc8615d-555b-42f4-8819-dca8e6572473"}

     c) Check order status:
        curl -H 'X-Project-Id:12345' http://localhost:9311/v1/orders/7bc8615d-555b-42f4-8819-dca8e6572473

        {"status": "ERROR", "updated": "2016-04-08T19:31:34.448945", "created": "2016-04-08T19:31:34.280043", "order_ref": "http://localhost:9311/v1/orders/7bc8615d-555b-42f4-8819-dca8e6572473", "meta": {"name": "ordered rsacontair1", "algorithm": "rsa", "mode": "cbc", "passphrase": "password", "bit_length": 2048, "expiration": null}, "error_status_code": "500", "error_reason": "Process TypeOrder failure seen - please contact site administrator.", "type": "asymmetric"}