[RPC] Add fail-guard for termination time exception

[tqchen]

This PR adds fail-guard to reduce error messages thrown during process termination time. Such error won't trigger test error but will bring extra message during exit time.

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[echuraev]

In general LGTM. One minor question. I didn't see any error messages during RPC termination before. In which scenario, I'll be able to get these import errors?

[tqchen]

This would be some rare case that have to do with shutdown orders likely in multi processing. I have observed some flaky ones during purest

[tqchen]

Example snippet

Exception ignored in: <function Server.__del__ at 0x7fb012d9add0>
Traceback (most recent call last):
  File "/home/tqchen/github/relax/python/tvm/rpc/server.py", line 502, in __del__
  File "/home/tqchen/github/relax/python/tvm/rpc/server.py", line 498, in terminate
  File "/home/tqchen/github/relax/python/tvm/contrib/popen_pool.py", line 145, in kill
  File "/home/tqchen/github/relax/python/tvm/contrib/popen_pool.py", line 43, in kill_child_processes
ImportError: sys.meta_path is None, Python is likely shutting down

[echuraev]

Thank you for the clarification!