Raise an exception when accessing a domain as subdomain

We were returning `nil` in this case, meaning we would be accessing the
main tenant in this situation instead of returning "Not Found".

app/models/tenant.rb

@@ -30,7 +30,11 @@ class Tenant < ApplicationRecord
       host_domain = allowed_domains.find { |domain| host == domain || host.ends_with?(".#{domain}") }
       schema = host_without_www.sub(/\.?#{host_domain}\Z/, "").presence
 
-      schema unless find_by_domain(schema)
+      if find_by_domain(schema)
+        raise Apartment::TenantNotFound
+      else
+        schema
+      end
     end
   end
 

spec/models/tenant_spec.rb

@@ -86,10 +86,10 @@ describe Tenant do
       expect(Tenant.resolve_host("saturn.consul.dev")).to eq "saturn"
     end
 
-    it "returns nil when a domain is accessed as a subdomain" do
+    it "raises an exception when a domain is accessed as a subdomain" do
       insert(:tenant, :domain, schema: "saturn.dev")
 
-      expect(Tenant.resolve_host("saturn.dev.consul.dev")).to be nil
+      expect { Tenant.resolve_host("saturn.dev.consul.dev") }.to raise_exception(Apartment::TenantNotFound)
     end
 
     it "returns nested subdomains when there's a subdomain-type tenant with nested subdomains" do