Everywhere: Add Error class for generic error reporting

The Lexer can now also report errors.

src/ast.h

@@ -23,7 +23,6 @@ public:
 	template<typename T>
 	bool fastIs() const = delete;
 
-	virtual bool isError() const { return false; }
 	virtual bool isVector() const { return false; }
 	virtual bool isHashMap() const { return false; }
 	virtual bool isList() const { return false; }
@@ -35,21 +34,6 @@ public:
 
 // -----------------------------------------
 
-class Error final : public ASTNode {
-public:
-	Error(const std::string& error);
-	virtual ~Error() = default;
-
-	virtual bool isError() const override { return true; }
-
-	const std::string& error() const { return m_error; }
-
-private:
-	std::string m_error;
-};
-
-// -----------------------------------------
-
 // []
 class Vector final : public ASTNode {
 public:
@@ -159,9 +143,6 @@ private:
 // -----------------------------------------
 
 // clang-format off
-template<>
-inline bool ASTNode::fastIs<Error>() const { return isError(); }
-
 template<>
 inline bool ASTNode::fastIs<Vector>() const { return isVector(); }