prepare-root: tweak log messages to clarify errors

This rewords errors and log messages in the functions which take care
of preparing sysroot in initramfs.
Depending on the boot flow, it is possible to reach this logic
with a sysroot mounted (unexpectedly) as read-only.
In that case, let's clearly point out the problematic mountpoint.

src/switchroot/ostree-prepare-root.c

@@ -189,6 +189,10 @@ main(int argc, char *argv[])
         err (EXIT_FAILURE, "usage: ostree-prepare-root SYSROOT");
       root_arg = argv[1];
     }
+#ifdef USE_LIBSYSTEMD
+  sd_journal_send ("MESSAGE=preparing sysroot at %s", root_arg,
+                    NULL);
+#endif
 
   struct stat stbuf;
   if (stat ("/proc/cmdline", &stbuf) < 0)
@@ -238,15 +242,20 @@ main(int argc, char *argv[])
    */
   const bool sysroot_readonly = sysroot_is_configured_ro (root_arg);
   const bool sysroot_currently_writable = !path_is_on_readonly_fs (root_arg);
-
 #ifdef USE_LIBSYSTEMD
-      sd_journal_send ("MESSAGE=sysroot configured read-only: %d, currently writable: %d", 
-                      (int)sysroot_readonly, (int)sysroot_currently_writable, NULL);
+  sd_journal_send ("MESSAGE=filesystem at %s currently writable: %d", root_arg,
+                   (int)sysroot_currently_writable,
+                   NULL);
+  sd_journal_send ("MESSAGE=sysroot.readonly configuration value: %d",
+                   (int)sysroot_readonly,
+                   NULL);
 #endif
+
   if (sysroot_readonly)
     {
       if (!sysroot_currently_writable)
-        errx (EXIT_FAILURE, "sysroot=readonly currently requires writable / in initramfs");
+        errx (EXIT_FAILURE, "sysroot.readonly=true requires %s to be writable at this point",
+              root_arg);
       /* Now, /etc is not normally a bind mount, but if we have a readonly
        * sysroot, we still need a writable /etc.  And to avoid race conditions
        * we ensure it's writable in the initramfs, before we switchroot at all.