Better error messages for neighbour and offset overflow.
Better error messages for neighbour and offset overflow.

Previous messages were just `"neighbour overflow"` and `"offset overflow"`. Now the messages are `value + " has no neighbour with offset " + offset` and `"offset from " + value + " to " + other + " cannot be represented as a 64 bit integer.", respectively.`

gijsleussink/ceylon,gijsleussink/ceylon,gijsleussink/ceylon,gijsleussink/ceylon,gijsleussink/ceylon