pre-check for write permissions before processing (Treeshop)

[e-t-k]

right now, if the person running fab doesn't have write permissions on downstream/secondary or primary/derived, fab process will get interrupted at the very end of a step, wasting a lot of time.
Instead, do a check at the beginning of fab process that all dirs are writable. Obviously this is vulnerable to (very slow) race conditions but it will catch the most common situation where a pre-existing dir was created by a different user.