mkinitrd shouln't quit when a kernel module is not found. It is entirely possible that the feature is builtin.
Created attachment 58046 [details] A patch to skip a kernel module.
I disagree -- many (certainly most new users) need to see this as an error condition. I've added a --builtin option which lets you tell mkinitrd certain modules are built into the kernel, so "--builtin ext3" will stop it from looking for ext3.o (and jbd.o as well). This isn't quite the same as what you wanted, but hopefully it will work well enough. Change in mkinitrd 3.3.11
I prefer something like --bultin-all so that I don't have to change the script when my kernel changes.