When starting Qt GUI sidechains (like testchain) the launcher does not know about syncing mainchain block cache

[CryptAxe]

When the Qt based sidechains like testchain start, they contact mainchain and synchronize a cache of mainchain block information. Since the launcher doesn't know about this it looks like it has frozen after hitting run on testchain until the mainchain block cache is synchronized.

We could check for this error message:

{ "result": <null>, "error": { "code": -28, "message": "Updating mainchain block cache..." }, "id": 1 }

or just wait for a minute, as syncing is pretty fast.

[CryptAxe]

Update: I tried to reproduce this again today, and it didn't freeze up and everything worked correctly. So this issue only happens sometimes but I haven't figured out why yet.

[CryptAxe]

We also may want to wait to start the sidechains anyway: #45