improve error message in NativeModuleRegistryBuilder.java (#26467)

Summary:
## Motivation

I have seen a spike in users reporting this error. Unfortunately I did not receive any repros that would confirm this, but my hypothesis is that they ran into situation when `new XYZPackage()` was present in `getPackages()` method and then the CLI kicked in with autolinking and they were left with this incomplete error.

someone more knowledgeable of autolinking should review this.
Pull Request resolved: #26467

Differential Revision: D17661242

Pulled By: cpojer

fbshipit-source-id: 63dfcd85a0d41d85a0dd52f84ab16cb7ceb64ba2

Author: vonovak

ReactAndroid/src/main/java/com/facebook/react/NativeModuleRegistryBuilder.java

@@ -52,7 +52,8 @@ public void processPackage(ReactPackage reactPackage) {
                   + name
                   + " tried to override "
                   + existingNativeModule.getClassName()
-                  + ". Check the getPackages() method in MainApplication.java, it might be that module is being created twice. If this was your intention, set canOverrideExistingModule=true");
+                  + ". Check the getPackages() method in MainApplication.java, it might be that module is being created twice. If this was your intention, set canOverrideExistingModule=true. "
+                  + "This error may also be present if the package is present only once in getPackages() but is also automatically added later during build time by autolinking. Try removing the existing entry and rebuild.");
         }
         mModules.remove(existingNativeModule);
       }