main/postmarketos-base/devmappings.initd: add some error handling

Hi,

I'm currently porting pmOS to the HTC Desire Z (htc-vision). That device doesn't boot, because it can't mount the root partition in read/write mode. After some investigation, I found that this is because it's missing the proper device nodes in /dev/mapper. These are supposed to be created in the devmappings init script. That script loops through the flash partitions and runs kpartx -afs "$partition" on those partitions with two subpartitions. On the aforementioned device, this command fails with a non-zero exit code when run on the /dev/mmcblk0p4 partition, but the pmOS_root and pmOS_boot partitions are located on /dev/mmcblk0p25. Since OpenRC apparently runs the script in -e mode, it aborts the loop and never reaches /dev/mmcblk0p25.

This patch fixes that issue.