Exit with a user-friendly message instead of tripping an assert

if vm_activate_cpu(..) fails when called from fbsdrun_addcpu(..)

MFC after: 1 week
PR: 203884
Reviewed by: grehan
Submitted by: William Orr <will@worrbase.com>

usr.sbin/bhyve/bhyverun.c

@@ -261,7 +261,8 @@ fbsdrun_addcpu(struct vmctx *ctx, int fromcpu, int newcpu, uint64_t rip)
 	 * with vm_suspend().
 	 */
 	error = vm_activate_cpu(ctx, newcpu);
-	assert(error == 0);
+	if (error != 0)
+		err(EX_OSERR, "could not activate CPU %d", newcpu);
 
 	CPU_SET_ATOMIC(newcpu, &cpumask);