Improvements to initialization logs and monitoring

[istreeter]

We had a problem where a loader could not move beyond the initialization block. The log messages and error messages made it hard to see exactly where the error happened. There are a few small changes we can make:

Log a message when the target check is successful. The [target check]https://github.com/snowplow/snowplow-rdb-loader/blob/4.2.1/modules/loader/src/main/scala/com/snowplowanalytics/snowplow/rdbloader/loading/TargetCheck.scala#L36] is the thing that just runs SELECT 1. Currently we log a message on a failure, but I think it would be helpful to see in the logs that the loader has successfully connected to the warehouse.

Log a message if there are no changes needed to the manifest table. Referring to this line. It will help to know when the manifest initilialization step is complete and the flow of execution has moved onto the next step.

The alert error message should be more specific where the error occurred. Currently we wrap the entire initialization block in an error handler. Which means the alert we send is simply "Exception from init block" (here). I think we need to wrap each step of the initialization separately, so the alert error messages can be more helpful like "Error waiting for target to be ready" or "Error initializing manifest table".