Log a warning if the homeserver is unreachable at startup

[DMRobertson]

Closes #286.

Tested as follows (see line 4):

Sync v3 [0.99.10] (6fe9b18)
Debug=true LogLevel= MaxConns=0
Starting prometheus listener on :6060
13:23:33 WRN Could not contact upstream homeserver. Is SYNCV3_SERVER set correctly? error="Get \"http://localhost:8888/_matrix/client/versions\": dial tcp [::1]:8888: connect: connection refused" dest=http://localhost:8888
2023/09/26 13:23:33 OK   20230728114555_device_data_drop_id.sql (7.48ms)
2023/09/26 13:23:33 OK   20230802121023_device_data_jsonb.go (13.85ms)
2023/09/26 13:23:33 OK   20230814183302_cbor_device_data.go (9.01ms)
2023/09/26 13:23:33 OK   20230822180807_bogus_snapshot_cleanup.go (7.64ms)
2023/09/26 13:23:33 OK   20230913120537_events_missing_previous.sql (7.15ms)
2023/09/26 13:23:33 goose: successfully migrated database to version: 20230913120537
13:23:33 INF creating handler
13:23:33 INF retrieved global snapshot from database
13:23:33 INF listening on 0.0.0.0:8844
13:23:33 INF StartV2Pollers num_devices=0 num_fail_decrypt=0
13:23:33 INF StartV2Pollers finished

Or for another example:

$ SYNCV3_SERVER=http://example.com SYNCV3_DB="user=$(whoami) dbname=syncv3_test sslmode=disable" SYNCV3_SECRET=secret SYNCV3_BINDADDR=0.0.0.0:8844 ./syncv3
Sync v3 [0.99.10] (7111ab1)
Debug=false LogLevel= MaxConns=0
13:28:24 WRN Could not contact upstream homeserver. Is SYNCV3_SERVER set correctly? error="/versions returned HTTP 404" dest=http://example.com
2023/09/26 13:28:24 goose: no migrations to run. current version: 20230913120537
13:28:24 INF creating handler
13:28:24 INF retrieved global snapshot from database
13:28:24 INF listening on 0.0.0.0:8844
13:28:24 INF StartV2Pollers num_devices=0 num_fail_decrypt=0
13:28:24 INF StartV2Pollers finished

[kegsay]

Why bother parsing when we do nothing with it? Just waiting for a 200 OK is enough.

[DMRobertson]

I'm sanity checking that you're receiving a proper Matrix /versions response and not just e.g. some Apache "it works" webpage.