fix(parsing/cmdgroup): report error the same way as everywhere else, prevent double report

before:
```
$ ()
minishell: syntax error near unexpected token `)'
minishell: syntax error near unexpected token `)'
```
after:
```
$ ()
minishell: syntax error near unexpected token `)'
```
This probably fixes a few bugs as well, but I didn't look too hard. It just
seemed nicer to have a consistent way to report errors.

test.sh

@@ -635,4 +635,13 @@ minishell: syntax error near unexpected token `|'
 2
 EOF
 
+when_run <<EOF "unclosed cmdgroup"
+(echo hi
+echo \$?
+EOF
+expecting <<"EOF"
+minishell: syntax error near unexpected token `newline'
+2
+EOF
+
 finalize