Handle OS Permission Errors

[nleanba]

When retrieving or creating files, deno runs into permission problems (linux filesystem)

This can be solved by one-time running

docker exec -u 0 [name of backend-container] chmod -R a+rw ./blobs/ rdf.ttl

This should be fixed automatically somehow when updating to a deno version of the package.

Note that the deno backend will return a message instruction to execute this command, but this message has to be seen -- it is useless when interacting via the cockpit.

Also, the message as sent by the backend only sets +r, but +w is necessary for document creation.