Copy, prune operations on non-existent storage IDs do not fail

Please describe what you are doing to trigger the bug:

Copy or prune non-existent -id.

Please describe what you expect to happen (but doesn’t):

Fail with error message.

Please describe what actually happens (the wrong behaviour):

New, empty snapshot id is created in source storage and command completes successfully.

Running copy command from /share/CACHEDEV1_DATA/.qpkg/Duplicacy/.duplicacy-web/repositories/localhost/all
Options: [-log copy -from NAS0 -to Backblaze-B2 -id non-exist]
2020-12-15 10:21:52.349 INFO STORAGE_SET Source storage set to /share/TR-004/Duplicacy/storage
2020-12-15 10:21:52.356 INFO STORAGE_SET Destination storage set to b2://kgorlen-NAS0
2020-12-15 10:21:52.762 INFO BACKBLAZE_URL download URL is: https://f000.backblazeb2.com
2020-12-15 10:21:53.462 INFO SNAPSHOT_COPY Nothing to copy, all snapshot revisions exist at the destination.

Running prune command from /share/CACHEDEV1_DATA/.qpkg/Duplicacy/.duplicacy-web/repositories/localhost/all
Options: [-log prune -storage Backblaze-B2 -keep 0:1800 -keep 7:30 -keep 1:7 -id non-exist -dry-run]
2020-12-15 10:35:11.080 INFO STORAGE_SET Storage set to b2://kgorlen-NAS0
2020-12-15 10:35:11.465 INFO BACKBLAZE_URL download URL is: https://f000.backblazeb2.com
2020-12-15 10:35:12.035 INFO RETENTION_POLICY Keep no snapshots older than 1800 days
2020-12-15 10:35:12.035 INFO RETENTION_POLICY Keep 1 snapshot every 7 day(s) if older than 30 day(s)
2020-12-15 10:35:12.035 INFO RETENTION_POLICY Keep 1 snapshot every 1 day(s) if older than 7 day(s)
2020-12-15 10:35:26.125 INFO FOSSIL_COLLECT Fossil collection 1 found
2020-12-15 10:35:26.125 INFO SNAPSHOT_INACTIVE Ignore snapshot Joint-Archive whose last revision was created 7 days ago
2020-12-15 10:35:26.125 INFO FOSSIL_POSTPONE Fossils from collection 1 can't be deleted because deletion criteria aren't met
2020-12-15 10:35:26.125 INFO SNAPSHOT_NONE No snapshot to delete

Some storages (like S3 and B2) don’t have the notion of directories, so it is impossible to tell an invalid repository id from a valid one with zero revisions.