Give trains pathfinding messages more meaning.

1. If there's a small gap in the rails, the message should read "Destination station not connected to current network," not generic "Train cannot reach destination."
I had a small gap in the rails, but I rebuilt a station a few times because I thought the station design was the problem.
2. If you click on a train that's having pathfinding issues, the train pathfinding overlay should start. It will be the same as for trucks.
I didn't know that trains have an overlay that can help me.
3. Right now, the train dialog box has a text box with the name of the station the train goes to. If there's an error, the station name is immediately hidden by the "searching for path" text. Don't do that.
I had a problem with pathfinding to the station group. I couldn't figure out which specific station was causing the issue.
4. "Arrival conditions not met" doesn't help me. A list of conditions and their states would be useful.

#3 especially. There's no point in showing the same error message twice, and debugging would be MUCH easier if you knew which station it was trying and failing to reach.