Improved error handling

[gecoombs]

Some recent bug reports have highlighted that error messages from PolarRoute aren't always clear or point to the wrong problem (e.g. #307 ). As part of the operationalisation process we should aim to have the code produce errors that are easy to understand, even for those who aren't active developers. This is especially important when it is a case where we expect the algorithm to fail (e.g. completely inaccessible waypoints, currents stronger than the vessel etc.).

We should also look at producing partial output if one particular part of the code fails. Currently if you plan multiple routes and one has an error then all of the other routes are lost even if they had run successfully.