Raise error for unterminated string literal

lib/chervil/lexer.rb

         delimiter = current_char
         advance
         string = String.new
-        until current_char == delimiter
+        until current_char == delimiter || current_char.nil?
           string << current_char
           advance
         end
 
         if current_char.nil?
-          raise "Unterminated string"
+          return Error.new("Unterminated string")
         end
 
         advance

spec/lexer_spec.rb

       expect(lexer.get_next_token).to eq(Token.new(:boolean, true))
       expect(lexer.get_next_token).to eq(Token.new(:boolean, false))
     end
+
+    it 'returns an error given an unterminated string' do
+      lexer = Lexer.new('"hello world')
+      expect(lexer.get_next_token).to eq(Error.new("Unterminated string"))
+    end
   end
 end