Exit group coordinator if member pid gone

[fmcgeough]

Fixes #607.

Terminate Group Coordinator if MemberPid is no longer alive when assignments_revoked, assign_partitions
or assignments_received must be called.

This is a first attempt to explore how to fix the issue raised in #607.

[zmstone]

Thank you for the PR @fmcgeough
But this will not eliminate the race condition completely.
this should:

try
  MemberModule:assignments_revoked(MemberPid)
catch
  exit:{noproc,{gen_server,call,[MemberPid | _]}}} ->
    exit({shutdown, member_down})
end,

to avoid having to repeat for all three calls, a macro can be better:

-define(CALL_MEMBER(MemberPid, EXPR),
  try
    EXPR
  exit:{noproc,{gen_server,call,[MemberPid | _]}}} ->
    exit({shutdown, member_down})
  end).

and use it like this: ?CALL_MEMBER(MemberPid, MemberModule:assignments_revoked(MemberPid))

[fmcgeough]

I've modified the PR to catch the exception rather than attempting to check if the process is alive. I'd appreciate any suggestions on how to do unit testing for this change.

[zmstone]

Thank you for the PR @fmcgeough