Improve errors a little bit

bin/chervil

       break if line == "(exit)"
       lexer = Chervil::Lexer.new(line)
       tree = Chervil::Parser.new(lexer).parse
+
+      if tree.is_a? Chervil::Error
+        puts tree
+        next
+      end
+
       results = Chervil::Interpreter.new(tree, env).interpret
       results.each { |result| puts result unless result.nil? }
     end

lib/chervil/parser.rb

 module Chervil
-  class Parser def initialize(lexer)
+  class Parser
+    def initialize(lexer)
       @lexer = lexer
       @tree = Array.new
       @current_token = @lexer.get_next_token
+      @error = nil
     end
 
     def parse
       until @current_token.type == :eof
         @tree << expr
+        return @error unless @error.nil?
       end
       @tree
     end
       elsif @current_token.type == :lparen
         application
       else
-        raise "Unexpected token #{@current_token.type}: #{@current_token.value}"
+        @error = Error.new("Unexpected token #{@current_token.type}: #{@current_token.value}")
       end
     end
 
       expression = expr
 
       args = Array.new
-      until @current_token.type == :rparen
+      until @current_token.type == :rparen || @current_token.type == :eof
         args << expr
       end
 
         @current_token = @lexer.get_next_token
         token
       else
-        raise "Expected #{type} but got #{@current_token.type}"
+        @error = Error.new("Expected #{type} but got #{@current_token.type}")
       end
     end
   end

spec/parser_spec.rb

         )
       )
     end
+
+    it 'returns a syntax error object instead of raising' do
+      expect(parse('(+ 1')).to eq(Error.new("Expected rparen but got eof"))
+      expect(parse('1)')).to eq(Error.new("Unexpected token rparen: )"))
+    end
   end
 end