throw exception when user not found

service/src/main/java/com/telephoners/krakyournet/ctf/exceptions/UserNotFoundException.java

+package com.telephoners.krakyournet.ctf.exceptions;
+
+import javax.ws.rs.WebApplicationException;
+import javax.ws.rs.core.Response;
+
+public class UserNotFoundException extends WebApplicationException
+{
+    public UserNotFoundException() {
+    super(Response.status(Response.Status.NOT_FOUND)
+        .entity("User not found.").type("text/plain").build());
+    }
+}

service/src/main/java/com/telephoners/krakyournet/ctf/repositories/UsersRepository.java

@@ -3,6 +3,7 @@ package com.telephoners.krakyournet.ctf.repositories;
 import com.google.inject.Inject;
 import com.google.inject.Singleton;
 import com.telephoners.krakyournet.ctf.beans.User;
+import com.telephoners.krakyournet.ctf.exceptions.UserNotFoundException;
 import com.telephoners.krakyournet.ctf.providers.MessageDigestProvider;
 import io.dropwizard.auth.basic.BasicCredentials;
 import org.apache.commons.codec.binary.Hex;
@@ -24,7 +25,10 @@ public class UsersRepository extends Repository<User>
     {
         return datastore.createQuery(User.class)
                 .field("name").equal(username)
-                .get();
+                .asList()
+                .stream()
+                .findFirst()
+                .orElseThrow(UserNotFoundException::new);
     }
 
     public User authenticateUser(BasicCredentials basicCredentials)