CU-86949yar7: Add logged warning for when multiprocessing fails due to being out of memory

[mart-r]

The underlying issue manifested in the following way:

When calling multiprocessing_batch_char_size the output would come out completely empty. Nothing logged as to what happened or why.

This is because the machine in question has 128GB of RAM and the default minimum expected RAM is 10%. And the machine happened to have around 6GB of free memory. Which is less than 5% (and less than the minimum required).
The solution for the user is to specify a smaller min_free_memory value when calling the method (e.g 0.005 in this case for around 500MB).

So what this PR does:

• Adds logged warning for when this happens along with suggestion for a fix
• Adds and uses new argument min_free_memory_size where users can specify a human-readable minimum memory amount

This way, in future cases, we'll know what the issue is and how to fix it.

[tomolopolis]

Task linked: CU-86949yar7 Multiprocessing fails with no indication as to why

[tomolopolis]

lgtm