Added more tests

src/errors/InvalidInstruction.go

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