More informative error functions for when integration fails to converge in fi_pool_cluster

[AngusMcLure]

Currently there is an error message, which is ok, and then a whole lot of printing of values to the screen. This could be tidied so as to print a tidier version of the same data, e.g. even creating a named list with the same outputs, and printing these may be more readable.

[AngusMcLure]

Also need to do something about the plots to screen. This has been useful for debugging in the past, but a user is probably never going to want to do this, so it can probably just come out (easy enough to put back if needed?)