Clarify difference between ER_CONNECTION_KILLED (1927) and error 2013

While writing MTR tests of procedure code, it's sometimes necessary to expect the current connection/thread to be killed.

I've been using the following in my MTR .test files:

--error ER_CONNECTION_KILLED

call PROCEDURE_THAT_WILL_CAUSE_THIS_CONNECTION_TO_DIE();

However, this occasionally results in tests failing

mysqltest: At line X, PROCEDURE_THAT_WILL_CAUSE_THIS_CONNECTION_TO_DIE() failed with wrong errno 2013: 'Lost connection to MySQL server during query', instead of 1927...

• The error ER_CONNECTION_KILLED (= 1927) is #define d in mysqld_error.h (https://github.com/mariadb-corporation/mariadb-connector-c/blob/HEAD/include/mysqld_error.h#L918), but the error 2013 is not named anywhere in the server code.
• The error 2013 is not defined anywhere that I can find in the MariaDB server repository; however it is described as a "client error" in
  https://github.com/mariadb-corporation/mariadb-connector-c/blob/HEAD/libmariadb/ma_errmsg.c#L43).
• Many existing MTR tests use --error 2013, ER_CONNECTION_KILLED (e.g. main/processlist.test) but others use only --error ER_CONNECTION_KILLED (e.g. main/kill.test), or only --error 2013.

It would be useful to have clarity on when/why an MTR test can sometimes fail with error 2013 instead of error 1927/ER_CONNECTION_FAILED.

• Is this a consequence of server- vs client-side timing of when the lost connection is detected?
• Is there a way to ensure we consistently get only one result, and not the other?
• If the variation is truly unavoidable, shouldn't all tests with --error ER_CONNECTION_KILLED also allow for --error 2013?