Apply suggestions from code review

Author: baywet

src/main/java/com/microsoft/graph/http/GraphError.java

@@ -91,6 +91,7 @@ protected String transformErrorCodeCase(@Nonnull final String original) {
      * Makes a deep copy of this GraphError
      * @return The copy of this GraphError
      */
+    @Nonnull
     public final GraphError copy() {
         GraphError errorCopy = new GraphError();
         errorCopy.message = this.message;

src/main/java/com/microsoft/graph/http/GraphErrorResponse.java

@@ -76,6 +76,7 @@ public final AdditionalDataManager additionalDataManager() {
      * Make and return a deep copy of this GraphErrorResponse.
      * @return The copy of this GraphErrorResponse.
      */
+    @Nonnull
     public GraphErrorResponse copy() {
         GraphErrorResponse responseCopy = new GraphErrorResponse();
         responseCopy.additionalDataManager = this.additionalDataManager;

src/main/java/com/microsoft/graph/http/GraphInnerError.java

@@ -65,6 +65,7 @@ public class GraphInnerError {
      * Make a deep copy this GraphInnerError, this includes all it's subsequent InnerError's.
      * @return The copy of this InnerError
      */
+    @Nonnull
     public final GraphInnerError copy() {
         GraphInnerError innerErrorCopy = new GraphInnerError();
         innerErrorCopy.code = this.code;