add more checks to visualisation.vector_plot and test checks

Nobody has claimed this yet.

At the moment there are a number of issues with the vector_plot function

1. it checks whether there is a single X and Y coord but then doesn't raise an exception
2, it doesn't check whether we have u and v winds (the names of these will be different depending on whether the data is on a rotated pole or not and whether they have been rotated already). At the moment, a user could pass U and V the wrong way around or even pass temperature and pressure and no error would be raised.

Start at visualisation.vector_plot and inspect its existing coordinate checks and handling of wind variables for rotated-pole and already-rotated data. Add tests covering invalid coordinate and variable inputs, including reversed or non-wind variables. Done means vector_plot raises appropriate exceptions for invalid inputs without changing valid plotting behavior.

Written by the indexing model from the issue text.