fix(grpc): ensure connection errors show onscreen

Fix an issue that was preventing grpc connection errors from showing
after a failed connection to a remote node.

Re-throw the error from `startLightningWallet` after logging it so that
it can be handled properly afterwards.

app/lib/zap/controller.js

@@ -284,6 +284,7 @@ class ZapController {
       this.sendMessage('lightningGrpcActive')
     } catch (err) {
       mainLog.warn('Unable to connect to Lighitnng gRPC interface: %o', err)
+      throw err
     }
   }