Allow to configure different aspect ratios of tiles.

For example for Plate Carree which has a 2x1 ratio
or OSGB which can use a 1x2 ratio
Add a OSGB36 projection to renderd

Add some tests for tile number to projected coordinate bounds
Try and be more helpful with error messages if an exception is thrown while saving.

Also don't exit renderd entirely if an exception is thrown during saving.