dde-file-manager shows a "possible damage" warning for a stick that mounted fine

Summary

Plugging in a read-only USB stick produces a scary warning from the file manager
that the device may be damaged. The device mounted successfully and needs no
repair. The warning is the visible end of a mount race inside Deepin: the file
manager's own mount request loses to the one udisks performs a fraction of a
second earlier, and the "failure" is reported to the user as a fault.

Environment

• Deepin 25 Community, dde-file-manager-daemon 6.x, udisks2
• Device: generic 512 MB USB stick, sdb, one vfat partition sdb1
• The stick has a hardware write-protect switch; in this test it was on, so
  udisksctl info -b /dev/sdb1 reports ReadOnly: true - correct, expected
  behaviour, not a fault

Steps to reproduce

1. Insert a USB stick that something else will auto-mount (read-only stick shown
   here, but the race is not read-only specific).
2. Watch the journal.

Observed

21:40:08.670  udisksd: block_devices/sdb: PartitionTable Properties Changed
21:40:08.745  udisksd: block_devices/sdb1 Filesystem: MountPoints=/media/wulf/60B7-D419
21:40:08.769  udisksd: UserspaceMountOptions=uhelper=udisks2
21:40:08.944  udisksd: Device /dev/sdb1 is already mounted at `/media/wulf/60B7-D419'
21:40:08.954  dde-file-manager-daemon: Block device mount failed -
              device "/org/freedesktop/UDisks2/block_devices/sdb1",
              error code dfmmount::DeviceError::kUDisksErrorAlreadyMounted,
              message "GDBus.Error:org.freedesktop.UDisks2.Error.AlreadyMounted:
              Device /dev/sdb1 is already mounted at `/media/wulf/60B7-D419'."

The user sees a warning implying possible device damage. The mount is fine:
lsblk shows sdb1 mounted at /media/wulf/60B7-D419, and udisks reports the
block device ReadOnly: true because of the switch.

Expected

• AlreadyMounted should be treated as success by the mount path, or suppressed
  silently - the user asked for a mount and a mount exists.
• If a device really is read-only, the message should say so ("the device is
  write-protected"), not imply damage.
• A device that is damaged should be distinguishable from an
  AlreadyMounted race; at present both end up as "may be damaged".

Notes

• The race is reproducible on every insert of this stick; whether the warning
  dialog is actually shown varies, but the failed mount is always logged.
• Two mount initiators are involved: udisks2 completes the mount ~200 ms before
  the file manager's request, which then fails with AlreadyMounted.
• The same stick mounted read-write (switch off) reports cleanly; so does a
  USB hard disk. Read-only media make it obvious because the resulting
  warning is even more alarming than a plain "failed to mount".

Suggestion

Treat org.freedesktop.UDisks2.Error.AlreadyMounted as a success in
dfmmount, and when a mount genuinely fails because the device is read-only,
report that specific condition rather than the generic damage warning.