kernel: Specify multiple kernel or initramfs in error message

It sometimes happens during osbuild that there are two kernel rpms
installed while setting up the initial error message, leading to
something like:

error: Finalizing rootfs: During kernel processing: Multiple vmlinuz- in usr/lib/ostree-boot

With the user unsure what the two kernel versions were. With this change
we print the kernel versions.

src/libpriv/rpmostree-kernel.cxx

@@ -89,15 +89,17 @@ find_kernel_and_initramfs_in_bootdir (int rootfs_dfd, const char *bootdir, char
       if (out_ksuffix ? g_str_has_prefix (name, "vmlinuz-") : g_str_equal (name, "vmlinuz"))
         {
           if (ret_kernel)
-            return glnx_throw (error, "Multiple vmlinuz%s in %s", out_ksuffix ? "-" : "", bootdir);
+            return glnx_throw (error, "Multiple vmlinuz%s in %s, occurrences '%s' and '%s/%s'",
+                               out_ksuffix ? "-" : "", bootdir, ret_kernel, bootdir, name);
           if (out_ksuffix)
             ret_ksuffix = g_strdup (name + strlen ("vmlinuz-"));
           ret_kernel = g_strconcat (bootdir, "/", name, NULL);
         }
       else if (g_str_equal (name, "initramfs.img") || g_str_has_prefix (name, "initramfs-"))
         {
           if (ret_initramfs)
-            return glnx_throw (error, "Multiple initramfs- in %s", bootdir);
+            return glnx_throw (error, "Multiple initramfs- in %s, occurrences '%s' and '%s/%s'",
+                               bootdir, ret_initramfs, bootdir, name);
           ret_initramfs = g_strconcat (bootdir, "/", name, NULL);
         }
     }