Fix Retrofit call cancellation triggering error

OkHttp throws a standard IOException with the message "Canceled"
whenever any call is cancelled, which is passed on by the base
ErrorHandlingCallback implementation to the ErrorUtils.getErrorMessage()
method, which in turn interprets it as a network connectivity issue, and
returns the appropriate error message. This commit fixes the issue by
adding a check in the ErrorHandlingCallback implementation’s failure
callback to verify that the call hasn’t been cancelled, before
delivering the error message.

OpenEdXMobile/src/main/java/org/edx/mobile/http/ErrorHandlingCallback.java

@@ -195,7 +195,7 @@ public void onFailure(@NonNull Call<T> call, @NonNull Throwable error) {
         if (progressCallback != null) {
             progressCallback.finishProcess();
         }
-        if (messageCallback != null) {
+        if (messageCallback != null && !call.isCanceled()) {
             messageCallback.onMessage(callTrigger.getMessageType(),
                     ErrorUtils.getErrorMessage(error, context));
         }