Handle RPC disconnection gracefully

[nazar-pc]

When RPC connection drops (node is shut down or instance), relayer prints tons of the following:

2021-11-05 07:12:56        RPC-CORE: getFinalizedHead(): BlockHash:: WebSocket is not connected
2021-11-05 07:12:56        RPC-CORE: getHeader(hash?: BlockHash): Header:: WebSocket is not connected
[1636089176740] ERROR: WebSocket is not connected
    Error: WebSocket is not connected
        at assert (/web/subspace/subspace-relayer/backend/node_modules/@polkadot/util/assert.cjs:34:11)
        at /web/subspace/subspace-relayer/backend/node_modules/@polkadot/rpc-provider/ws/index.cjs:491:26
        at new Promise (<anonymous>)
        at WsProvider.send (/web/subspace/subspace-relayer/backend/node_modules/@polkadot/rpc-provider/ws/index.cjs:489:12)
        at callWithRegistry (/web/subspace/subspace-relayer/backend/node_modules/@polkadot/rpc-core/bundle.cjs:258:42)
        at Observable._subscribe (/web/subspace/subspace-relayer/backend/node_modules/@polkadot/rpc-core/bundle.cjs:265:9)
        at Observable._trySubscribe (/web/subspace/subspace-relayer/backend/node_modules/rxjs/dist/cjs/internal/Observable.js:41:25)
        at /web/subspace/subspace-relayer/backend/node_modules/rxjs/dist/cjs/internal/Observable.js:35:31
        at Object.errorContext (/web/subspace/subspace-relayer/backend/node_modules/rxjs/dist/cjs/internal/util/errorContext.js:22:9)
        at Observable.subscribe (/web/subspace/subspace-relayer/backend/node_modules/rxjs/dist/cjs/internal/Observable.js:26:24)
2021-11-05 07:12:56        RPC-CORE: getFinalizedHead(): BlockHash:: WebSocket is not connected

Ideally it should just crash.