Set shell and shell options in fetch_markdown_files.sh

Summary:
Running the script gives no indication of it working or failing
other than exit code. Not only that, but there's no guarantee that it's run
using bash.

This patch fixes both using the same convention we use in the ABC repo.

Test Plan:

./fetch_markdown_files.sh ~/projects/bitcoin-abc

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7575