'conc_sys' test

Concurrent execution of SELECT involing at least two INFORMATION_SCHEMA
tables, DROP DATABASE statement and DROP TABLE statement could have
resulted in stalled connection for this SELECT statement.

The problem was that for the first query of a join there was a race
between select from I_S.TABLES and DROP DATABASE, and the error (no
such database) was prepared to be send to the client, but the join
processing was continued.  On second query to I_S.COLUMNS there was a
race with DROP TABLE, but this error (no such table) was downgraded to
warning, and thd->net.report_error was reset.  And so neither result
nor error was sent to the client.

The solution is to stop join processing once it is clear we are going
to report a error, and also to downgrade to warnings file system errors
like 'no such database' (unless we are in the 'SHOW' command), because
I_S is designed not to use locks and the query to I_S should not abort
if something is dropped in the middle.

No test case is provided since this bug is a result of a race, and is
timing dependant.  But we test that plain SHOW TABLES and SHOW COLUMNS
give a error if there is no such database or a table respectively.