feat: change hasone relation error message

The current hasone error message is not appropriate one.
So adds a better message.

lib/model.js

@@ -565,7 +565,9 @@ module.exports = function(registry) {
     if (ctx.result !== null) return cb();
 
     const fk = ctx.getArgByName('fk');
-    const msg = g.f('Unknown "%s" id "%s".', toModelName, fk);
+    const msg = fk ?
+      g.f('Unknown "%s" id "%s".', toModelName, fk) :
+      g.f('No "%s" instance(s) found', toModelName);
     const error = new Error(msg);
     error.statusCode = error.status = 404;
     error.code = 'MODEL_NOT_FOUND';

test/relations.integration.js

@@ -1810,6 +1810,11 @@ describe('relations - integration', function() {
       const url = '/api/customers/' + cust.id + '/profile';
       this.get(url)
         .expect(404, function(err, res) {
+          const expected = 'No "profile" instance(s) found';
+          expect(res.body.error.message).to.be.equal(
+            expected,
+          );
+          expect(res.body.error.code).to.be.equal('MODEL_NOT_FOUND');
           done(err);
         });
     });