Enforced consistent naming

src/errors/InvalidOperator.go

@@ -0,0 +1,13 @@
+package errors
+
+import "fmt"
+
+// InvalidOperator error is created when an operator is not valid.
+type InvalidOperator struct {
+	Operator string
+}
+
+// Error generates the string representation.
+func (err *InvalidOperator) Error() string {
+	return fmt.Sprintf("Invalid operator '%s'", err.Operator)
+}