Stdlib_unix: improve error message when failing to load stored_data

This MR aims to improve the error message when the store fails to load it's Stored_data.t. Such error may occur when the file permissions are wrong or when files are missing/corrupted.

This was particularly embarrassing when some user are running/importing/upgrading snapshots/nodes with different rights, without noticing it, resulting in the obscure error below.

Before:

ct 03 15:47:02.733: the store is in an inconsistent state:
Oct 03 15:47:02.733:   Error:
Oct 03 15:47:02.733:     Failed to load on-disk data: no corresponding data found in file /tmp/lapin/store/chain_NetXdQprcVkpa/status.
Oct 03 15:47:02.733: 
Oct 03 15:47:02.733: attempting to restore the store's consistency...
Oct 03 15:47:02.750: the consistency of the floating stores was restored
octez-node: Error:
              Failed to load on-disk data: no corresponding data found in file /tmp/lapin/store/chain_NetXdQprcVkpa/status.

Now:

• when wrong permission

Oct 03 16:21:08.416: the store is in an inconsistent state:
Oct 03 16:21:08.416:   Error:
Oct 03 16:21:08.416:     Failed to load on-disk data from /tmp/lapin/store/chain_NetXdQprcVkpa/status: in open "/tmp/lapin/store/chain_NetXdQprcVkpa/status": Permission denied.
Oct 03 16:21:08.416: 
Oct 03 16:21:08.416: attempting to restore the store's consistency...
Oct 03 16:21:08.430: the consistency of the floating stores was restored
octez-node: Error:
              Failed to load on-disk data from /tmp/lapin/store/chain_NetXdQprcVkpa/status: in open "/tmp/lapin/store/chain_NetXdQprcVkpa/status": Permission denied.

• when corrupted file

Oct 03 16:22:11.579: the store is in an inconsistent state:
Oct 03 16:22:11.579:   Error:
Oct 03 16:22:11.579:     Failed to load on-disk data from /tmp/lapin/store/chain_NetXdQprcVkpa/status: Data_encoding.Read_error(Not enough data).
Oct 03 16:22:11.579: 
Oct 03 16:22:11.579: attempting to restore the store's consistency...
Oct 03 16:22:11.587: the consistency of the floating stores was restored
octez-node: Error:
              Failed to load on-disk data from /tmp/lapin/store/chain_NetXdQprcVkpa/status: Data_encoding.Read_error(Not enough data).

Manually testing the MR

Create a data dir in sandbox. Alter the /tmp/lapin/store/chain_NetXdQprcVkpa/status file by:

• changing it's rights
• breaking it's data

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR