EventBase::runAfterDelay to throw an exception

EventBase::runAfterDelay to throw an exception

Summary:
EventBase::runAfterDelay to throw an exception in case it fails to
schedule a proper execution of the given callback
(instead of silently returning false)

it appears to be the right thing to do, since:
@davejwatson: Digging through the layers of code, it appears this can only fail if
epoll_ctl() with EPOLL_CTL_ADD fails. Ignoring libevent bugs, it looks
like the only relevant errors could be ENOMEM or ENOSPC. So
nonrecoverable

Test Plan: Unit tests

Reviewed By: anca@fb.com

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FB internal diff: D1805125

Signature: t1:1805125:1424927912:8bebb4c3b9f1fa189c0ce97b12cdb8f95dba97ae