Should be tolerant to missing tools folder

[NicoHood]

Imported from here: arduino/Arduino#5778 (comment)

The IDE also should be tolerant of a missing tools-builder folder:
https://bugs.archlinux.org/task/52377

How to reproduce:
you need to delete the described folder (or better move it somewhere else) and change platform.txt to point to the new loation. make sure your .arduino15 folder does not exist, otherwise the bug is not triggered (as it is not for me)