crimson/osd/main: catch exception thrown in the async() call

crimson/osd/main: catch exception thrown in the async() call

* use seastar::app_template::run() instead of
  seastar::app_template::run_deprecated() for returning int,
  instead of returning `void`. so the application can return
  int explicitly in the continuation passed to run(). more
  readable this way.
* wrap the all the block in run() in a giant try-catch block,
  so the exceptions thrown by the startup code can be captured
  and handled.
* do not capture the exceptions individually, in the try-catch
  block anymore. the outer catch block takes care of them.

this change improves the error handling when crimson-osd launches.

Signed-off-by: Kefu Chai <kchai@redhat.com>