improve message for rwlock error

[efiop]

https://discordapp.com/channels/485586884165107732/563406153334128681/667133061380571146

Current message:

ERROR: failed to reproduce 'device_download.dvc': 'data/device_modifier.parquet' is busy, it is being read by:
  (PID 28324): dvc repro device_train.dvc

even though I removed .dvc/lock file

[shcheklein]

consider adding the second troubleshooting section now that we have a good mechanism for this :)

It is not clear how to improve it 😅
@efiop , any specific suggestions?

[shcheklein]

@MrOutis I suppose in any way that would prevent us answering the same question on Discord again and again, right? :)

[efiop]

@MrOutis yep, at least a hint how to delete it. Same as we do with .dvc/lock