The do_check_io_limits() function returns false when I/O limits are
invalid but it doesn't set an Error to indicate why. The two
do_check_io_limits() callers duplicate error reporting. Solve this by
passing an Error pointer into do_check_io_limits().
Note that the two callers report slightly different errors: drive_init()
prints a custom error message while qmp_block_set_io_throttle() does
error_set(errp, QERR_INVALID_PARAMETER_COMBINATION).
QERR_INVALID_PARAMETER_COMBINATION is a generic error, see
include/qapi/qmp/qerror.h: