Implemented invalid character checks

src/errors/InvalidCharacter.go

@@ -0,0 +1,13 @@
+package errors
+
+import "fmt"
+
+// InvalidCharacter error is created when an invalid character appears.
+type InvalidCharacter struct {
+	Character string
+}
+
+// Error generates the string representation.
+func (err *InvalidCharacter) Error() string {
+	return fmt.Sprintf("Invalid character '%s'", err.Character)
+}