Unfortunately far from perfect. Efetch quite often fails with larger downloads and doesn't necessarily even spit out a warning or anything. I would download the GIs instead of fasta and then to begin with check that the number of downloaded GIs is the same than:

esearch -db nuccore -query "("2015/10/01"[Publication Date] : "2015/11/09"[Publication Date])" | xtract -element Count

Then I'd split the list of GIs with split to e.g. 500 lines per file and then loop over those..

for f in *.splitFile
do
    IDs=$(cat $f | tr "\n" "," | sed 's/,$//')
    epost -db nuccore -id $IDs | efetch -format fasta > $f.fna
done

In addition you need to build some kind of check for these batch downloads. E.g. the file should have as many headers as there were lines in the id file. All is great then as long as download didn't fail in the middle of the last sequence :)