Improved variable lifetime tracking

src/errors/UnusedVariable.go

@@ -0,0 +1,13 @@
+package errors
+
+import "fmt"
+
+// UnusedVariable error is created when a defined variable is never used.
+type UnusedVariable struct {
+	Name string
+}
+
+// Error generates the string representation.
+func (err *UnusedVariable) Error() string {
+	return fmt.Sprintf("Unused variable '%s'", err.Name)
+}