AK3: be clear when unpack_ramdisk fails because there is no ramdisk

- previously this was failing a bit too spectacularly for my liking since $comp would erroneously get set to "fopen: ramdisk.cpio failed with 2: No such file or directory"
- on devices where there is no ramdisk.cpio unless rooted with Magisk this will indicate better to developers working on the device that they should be using "OG AK" mode to avoid trying to unpack a ramdisk which may or may not exist

tools/ak3-core.sh

@@ -123,7 +123,11 @@ unpack_ramdisk() {
     mv -f ramdisk.cpio.gz ramdisk.cpio;
   fi;
 
-  comp=$($bin/magiskboot decompress ramdisk.cpio 2>&1 | head -n1 | cut -d[ -f2 | cut -d] -f1 | grep -v 'compressed');
+  if [ -f ramdisk.cpio ]; then
+    comp=$($bin/magiskboot decompress ramdisk.cpio 2>&1 | head -n1 | cut -d[ -f2 | cut -d] -f1 | grep -v 'compressed');
+  else
+    abort "No ramdisk found to unpack. Aborting...";
+  fi;
   if [ "$comp" -a "$comp" != "raw" ]; then
     mv -f ramdisk.cpio ramdisk.cpio.$comp;
     $bin/magiskboot decompress ramdisk.cpio.$comp ramdisk.cpio;
@@ -310,7 +314,7 @@ flash_boot() {
   fi;
 
   if [ ! -f boot-new.img ]; then
-    abort "Repacked image could not be found. Aborting...";
+    abort "No repacked image found to flash. Aborting...";
   elif [ "$(wc -c < boot-new.img)" -gt "$(wc -c < boot.img)" ]; then
     abort "New image larger than boot partition. Aborting...";
   fi;