Added more tests

src/errors/CompileErrors.go

@@ -1,5 +1,6 @@
 package errors
 
 var (
-	MissingAssignmentValue = &Base{"Missing assignment value"}
+	InvalidStatement   = &Base{"Invalid statement"}
+	MissingAssignValue = &Base{"Missing assignment value"}
 )

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)
+}