Show full error messages

[joeytalbot]

When a route fails, it would be very helpful to show the error message that explains why the route failed. E.g.

"Too short: journeys must be longer than 4 metres. (Your requested journey was 0 metres)."

This is especially needed for the batch routing, where a single failed route prevents the entire process from completing.

[Robinlovelace]

May be fixed by #35

[mvl22]

Use the 'errors' field in the /v2/batchroutes.jobdata output.

We recommend you output those to the terminal so the person running the batch job gets that summary of the errors.

[Robinlovelace]

Makes sense, that would definitely be an improvement. For some reason I thought the error message would be in the json field as valid json but that's a valid place for it to be.