Make nicer the error message when not enought user-space exists

Comments from Tim:

*If I set the maximum space for user to a certain values and my
simulation requires more space:
ERROR you only have 4123917kB if HDD available but the operation you
laucnhed midht require 8080488. Stopping execution
which make me believe that I do not have enough space on my hard drive.

• When we have the following errors:
  ERROR: You only have 3927126.0 kiloBytes of HDD available but the operation you launched might require 12928781. Stopping execution...
  Could we put that in Gigabytes?