api v2: Return errors to help diagnose faulty networks

[tahini]

In an ideal world, the data read by trRouting would be perfect, but the world being what it is, it sometimes happen that the data is corrupted. trRouting correctly detects it and adds an error in the log, something like for example

Index out of range while parsing connection for trip on line (
My Super Line (1000))

When that happens, there is simply no data available. Routing requests return that there is no connection, no service, no nothing.

It should return that the data is corrupt and ideally highlight the faulty line so that callers can handle it correctly and let the consumers know about potential problems.

[tahini]

cc @greenscientist Potential christmas gift for @tibobliss?