Don't allow binding to an unbound variable

lib/chervil/ast/definition.rb

     end
 
     def evaluate(env)
+      @value = value.evaluate(env) if value.is_a?(Identifier)
+      return @value if @value.is_a?(::Chervil::Error)
       env.set(@name.name, @value)
       nil
     end

lib/chervil/interpreter.rb

     end
 
     def interpret
-      @tree.map { |node| node.evaluate(@env) }
+      @tree.map do |node|
+        result = node.evaluate(@env)
+        return [result] if result.is_a?(Error)
+        result
+      end
     end
   end
 end

spec/ast/definition_spec.rb

       ).evaluate(env)
       expect(env.get("x")).to eq(AST::Number.new(5.0))
     end
+
+    it 'throws an error if bound to an unbound variable' do
+      env = Env.new
+      result = AST::Definition.new(
+        AST::Identifier.new("x"),
+        AST::Identifier.new("y"),
+      ).evaluate(env)
+      expect(result).to eq(
+        ::Chervil::Error.new("unbound variable y")
+      )
+    end
   end
 end