about 3/4 the way down
The most likely build errors you may encounter are those indicating that StopConnectors features have an empty geometry. For example, a common message is SourceName: StopConnectors, ObjectID: 232, Geometry is empty. This typically occurs when one of the Stops features and its associated StopsOnStreets feature are spatially coincident because the StopsOnStreets feature did not successfully snap to a nearby street. Consequently, the connector line generated between the two has no length.
That build error indicates that the transit stop has not successfully connected to a street feature. There are a couple of likely reasons for this:
I think you need to take a close look at both the locations of your transit stops and also your street data to determine which of the above cases is causing your problem. Once you've determined that, then you can figure out how to fix it.
If the problem is only for a few stops, and they're in an area where you don't expect them to be used for your analysis, then you can safely ignore the build errors. The rest of the network will continue to function. Otherwise, you will need to resolve the issue. You might need to edit the stop locations (or correct the original GTFS and start over), update your street data or use a different street dataset, or re-run Connect Network Dataset Transit Sources To Streets with a larger search tolerance.
To map the frequency of public transit service, you can use the Count Trips on Lines tool in the BetterBusBuffers toolbox: http://esri.github.io/public-transit-tools/BetterBusBuffers.html
However, this tool will not map the frequency of bus service on the streets. The lines produced by the tool are simple straight lines connecting adjacent stops. I have not found a good way to match these lines to the underlying street features, unfortunately.