fix: error message when cannot run 'datastore gc' or 'datastore repair'

[miparnisari]

Fix error message that appears when trying to run datastore gc or datastore repair on datastores that don't support them.

Before

$ docker run --name cockroachdb -d --rm -p 26257:26257 -p 8080:8080 cockroachdb/cockroach:latest start-single-node --insecure --max-offset=50ms 

$  go run ./cmd/spicedb/main.go datastore gc \
--datastore-engine cockroachdb \
--datastore-conn-uri "postgresql://root@localhost:26257/defaultdb?sslmode=disable"
...

ERR terminated with errors error="datastore of type *datastore.ctxProxy does not support garbage collection"                                                                                                     
exit status 1

After

$ docker run --name cockroachdb -d --rm -p 26257:26257 -p 8080:8080 cockroachdb/cockroach:latest start-single-node --insecure --max-offset=50ms 

$  go run ./cmd/spicedb/main.go datastore gc \
--datastore-engine cockroachdb \
--datastore-conn-uri "postgresql://root@localhost:26257/defaultdb?sslmode=disable"
...

ERR terminated with errors error="datastore of type 'cockroachdb' does not support garbage collection"
exit status 1

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.07%. Comparing base (c39bf5f) to head (1c0b6cc).

❌ Your project check has failed because the head coverage (59.07%) is below the target coverage (75.00%). You can increase the head coverage or adjust the target coverage.

❗ There is a different number of reports uploaded between BASE (c39bf5f) and HEAD (1c0b6cc). Click for more details.

HEAD has 28 uploads less than BASE

Flag	BASE (c39bf5f)	HEAD (1c0b6cc)
	54	26

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2609       +/-   ##
===========================================
- Coverage   79.42%   59.07%   -20.35%     
===========================================
  Files         455      413       -42     
  Lines       47156    43729     -3427     
===========================================
- Hits        37448    25827    -11621     
- Misses       6950    15411     +8461     
+ Partials     2758     2491      -267     

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[miparnisari]

TO DO: the tests aren't running 🫠