rest_router: stop catching Exception

catch more specific exceptions instead.

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
stringify.StringifyMixin.__str__: simplify

simplify the code a bit.
suggested by Isaku Yamahata.
no functional changes are intended.

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>