Added more logging; add variable to make pre-existing disk formatting easier to configure

Prints traceback when errors occur. Easier to figure out where the error actually occurred - too vague otherwise.

Get a list of disks that already exist. The force_reformat = True variable can be later tweaked (at least it'll be useful) or turned into a CLI arg. What it does: if it is True, all pre-existing 100G disks will be formatted as usual (the mkfs command), else they will be left as they are and attached to the corresponding instance. This gives the user an option to prevent loss of data.