Handle incorrect duration more elegantly
Handle incorrect duration more elegantly

The previous version of the LUTestDelay opcode relied on the utility
function complaining about the negative duration. As this function has
been removed for now, do the check ourselves, and issue a more
appropriate exception.

Signed-off-by: Hrvoje Ribicic <be5828fca31f89218048bec4189e30a581aa65da@google.com>
Reviewed-by: Petr Pudlak <9d19625f8ee825a2c398462ed335cbcf98f79c32@google.com>

andir/ganeti,ganeti-github-testing/ganeti-test-1,yiannist/ganeti,yiannist/ganeti,leshchevds/ganeti,dimara/ganeti,leshchevds/ganeti,apyrgio/ganeti,mbakke/ganeti,grnet/snf-ganeti,ganeti/ganeti,grnet/snf-ganeti,bitemyapp/ganeti,bitemyapp/ganeti,ganeti/ganeti,leshchevds/ganeti,apyrgio/ganeti,mbakke/ganeti,onponomarev/ganeti,ganeti/ganeti,andir/ganeti,mbakke/ganeti,yiannist/ganeti,dimara/ganeti,andir/ganeti,ganeti-github-testing/ganeti-test-1,onponomarev/ganeti