Added mock query conversion service

internal/usecases/convertquery/mock/mockconvertquery.go

 package mockconvertquery
 
+import "errors"
+
 // A Service implements the query convert usecase interface (mock)
 type Service struct {
+	throwError bool
 }
 
 // NewService creates a new query convert service (mock)
 func NewService() *Service {
-	return &Service{}
+	return &Service{
+		throwError: false,
+	}
 }
 
 // ConvertQuery returns a hard coded string message (mock)
 func (s *Service) ConvertQuery(jsonMsg *[]byte) (*string, error) {
 	mockQuery := "Query converted"
-	return &mockQuery, nil
+
+	if !s.throwError {
+		return &mockQuery, nil
+	}
+	return nil, errors.New("Failed to convert query")
+}
+
+// ToggleError decides whether the convert function throws an error
+func (s *Service) ToggleError() {
+	s.throwError = !s.throwError
 }