Improve error when submitting work from a closed client

[jrbourbeau]

Today if someone accidentally does client.submit or client.map with a closed client, they get a huge, not super meaningful error:

Exception: Tried sending message after closing.  Status: closed
Message: {'op': 'update-graph', 'graph_header': {'serializer': 'pickle', 'writeable': ()}, 'graph_frames': 
[b'\x80\x05\x95\x9bJ\x00\x00\x00\x00\x00\x00\x8c\x1edistributed.protocol.serialize\x94\x8c\x08ToPickle\x94\x93\x94)\x81\x94}\x94\x8c\x04data\x94\x8c\x13
dask.highlevelgraph\x94\x8c\x0eHighLevelGraph............... # this bytestring goes on for a long time

This PR update the error in this case to be more concise:

ClosedClientError: Client is closed. Can't send update-graph message.

[github-actions]

Unit Test Results

See test report for an extended history of previous test failures. This is useful for diagnosing flaky tests.

    27 files  ± 0      27 suites  ±0   11h 9m 46s ⏱️ - 4m 14s
 4 102 tests + 1   3 985 ✅ + 1    112 💤 ±0  5 ❌ +1 
51 426 runs  +12  49 120 ✅ +13  2 297 💤 ±0  9 ❌ ±0 

For more details on these failures, see this check.

Results for commit 9dfd158. ± Comparison against base commit 3f17ddb.

[jacobtomlinson]

This is a nice improvement! CI failures appear unrelated.