Added more tests

src/errors/VariableAlreadyExists.go

@@ -0,0 +1,13 @@
+package errors
+
+import "fmt"
+
+// VariableAlreadyExists is used when existing variables are used for new variable declarations.
+type VariableAlreadyExists struct {
+	Name string
+}
+
+// Error generates the string representation.
+func (err *VariableAlreadyExists) Error() string {
+	return fmt.Sprintf("Variable '%s' already exists", err.Name)
+}