Add note for failed logins on dev auth screen

Author: niftynei

app/src/main/java/com/zulip/android/activities/DevAuthActivity.java

@@ -5,6 +5,7 @@
 import android.os.Bundle;
 import android.support.v7.widget.LinearLayoutManager;
 import android.support.v7.widget.RecyclerView;
+import android.widget.Toast;
 
 import com.zulip.android.R;
 import com.zulip.android.networking.AsyncDevGetEmails;
@@ -73,6 +74,7 @@ public void onSuccess(Call<LoginResponse> call, Response<LoginResponse> response
                             @Override
                             public void onError(Call<LoginResponse> call, Response<LoginResponse> response) {
                                 // do nothing
+                                Toast.makeText(DevAuthActivity.this, R.string.login_failed, Toast.LENGTH_LONG).show();
                             }
 
                         });

app/src/main/res/values/strings.xml

@@ -88,4 +88,5 @@
     <string name="login_activity_toast_login_error">Unknown login error.</string>
     <string name="no_topic_in_message">(no topic)</string>
     <string name="message_copied">Message Copied</string>
+    <string name="login_failed">Unable to login</string>
 </resources>