mount: improve mountpoint error msgs, see #7496

saying "must be a writable directory" can distract
from the real root cause as seen in #7496.

so we better first check if the mountpoint is an
existing directory and if not, just tell that.

after that, we check permissions and if they are not
like required, tell that.

src/borg/archiver/mount_cmds.py

@@ -24,8 +24,12 @@ class MountMixIn:
             self.print_error("borg mount not available: no FUSE support, BORG_FUSE_IMPL=%s." % BORG_FUSE_IMPL)
             return self.exit_code
 
-        if not os.path.isdir(args.mountpoint) or not os.access(args.mountpoint, os.R_OK | os.W_OK | os.X_OK):
-            self.print_error("%s: Mountpoint must be a writable directory" % args.mountpoint)
+        if not os.path.isdir(args.mountpoint):
+            self.print_error(f"{args.mountpoint}: Mountpoint must be an **existing directory**")
+            return self.exit_code
+
+        if not os.access(args.mountpoint, os.R_OK | os.W_OK | os.X_OK):
+            self.print_error(f"{args.mountpoint}: Mountpoint must be a **writable** directory")
             return self.exit_code
 
         return self._do_mount(args)