Use MiB, GiB et al when the target backup partition is too small

[framps]

Given pibackup creates an image of the first two partitions and compresses the image later on I faced multiple time an error message which told me how much space is required and how much space is available because the target partition was too small. The space is displayed in bytes which usually is a huge number. I suggest to write the sizes in MiB or GiB which makes the sizes much more readable.