Improve error message for missing button (#1759)

Author: samtstern

auth/src/main/java/com/firebase/ui/auth/AuthUI.java

@@ -487,7 +487,6 @@ public SignInIntentBuilder createSignInIntentBuilder() {
             ANONYMOUS_PROVIDER,
             EmailAuthProvider.EMAIL_LINK_SIGN_IN_METHOD
     })
-
     @Retention(RetentionPolicy.SOURCE)
     public @interface SupportedProvider {
     }

auth/src/main/java/com/firebase/ui/auth/ui/idp/AuthMethodPickerActivity.java

@@ -223,7 +223,7 @@ private void populateIdpListCustomLayout(List<IdpConfig> providerConfigs) {
             final String providerId = providerOrEmailLinkProvider(idpConfig.getProviderId());
 
             if (!providerButtonIds.containsKey(providerId)) {
-                throw new IllegalStateException("No button found for auth provider: " + providerId);
+                throw new IllegalStateException("No button found for auth provider: " + idpConfig.getProviderId());
             }
 
             @IdRes int buttonId = providerButtonIds.get(providerId);