Add a note for unclosed delimiters

Currently, the parser doesn't give any context when it finds an unclosed
delimiter and it's not EOF. Report the most recent unclosed delimiter, to help
the user along.

Closes #10636
auto merge of #10641 : cmr/rust/close_delims, r=alexcrichton

Currently, the parser doesn't give any context when it finds an unclosed
delimiter and it's not EOF. Report the most recent unclosed delimiter, to help
the user along.

Closes #10636