Check configured GTFS data paths exist before commencing analysis

[carlhiggs]

We run various data checks to alert users of mis-configuration before commencing analysis, including whether configured data cannot be located (e.g. for study region boundaries, etc).

However, I just found that analysis can still proceed using a mis-configured GTFS folder path, as per post in #449 .

We should add in checks for GTFS folder path and configured GTFS feeds to alert users before commencing analysis, so this doesn't result in issues once GTFS analysis is commenced. Better to get a friendly warning sooner, than have a smooth analysis workflow interupted unexpectedly requiring recommencing of analysis or manual continuation of subprocesses.