The Server executed EXPLAIN FORMAT=JSON of some malformed queries improperly.

Fix:
Both JOIN::explain() and st_select_lex_unit::explain() functions
have been modified to return error statuses instead of void, as
well as the calling code has been changed to take into account
that error states and to abort its execution on error before
unneeded calls to Explain*::end_context().
That protects underlying EXPLAIN implementation from doing output
of half-initialized intermediate structures.