Description of problem:

This is visible from hammer too:
hammer -v -u admin -p changeme --output=csv realm create --name="foo" --realm-proxy-id="P0D2T027xd" --realm-type="Red Hat Identity Management"'

ERROR 2018-07-06T13:33:23 API] 500 Internal Server Error
[ERROR 2018-07-06T13:33:23 Exception] PG::ForeignKeyViolation: ERROR:  insert or update on table "realms" violates foreign key constraint "realms_realm_proxy_id_fk"
DETAIL:  Key (realm_proxy_id)=(0) is not present in table "smart_proxies".
: INSERT INTO "realms" ("name", "realm_type", "realm_proxy_id", "created_at", "updated_at") VALUES ($1, $2, $3, $4, $5) RETURNING "id"
Could not create the realm:
  PG::ForeignKeyViolation: ERROR:  insert or update on table "realms" violates foreign key constraint "realms_realm_proxy_id_fk"
  DETAIL:  Key (realm_proxy_id)=(0) is not present in table "smart_proxies".
  : INSERT INTO "realms" ("name", "realm_type", "realm_proxy_id", "created_at", "updated_at") VALUES ($1, $2, $3, $4, $5) RETURNING "id"
[ERROR 2018-07-06T13:33:23 Exception] 


- I can see 2 issues here:
 #1 hammer does not validate the parameter type (this used to work in 6.3 therefore it's a regression)
 #2 there is an improper error handling (ther ecould be a 404 instead of ISE) - this one might be present in 6.3 as well.

Version-Release number of selected component (if applicable):
6.4.0-11

How reproducible:
always

Steps to Reproduce:
1. POST to /realms endpoint with specifying invalid realm proxy id