Make download_holopix50k.sh executable.

[ndahlquist]

• Sets the executable bit of download_holopix50k.sh, since it's conventional to be able to execute scripts.
• Updated the readme to call the script like ./scripts/download_holopix50k.sh
• Called set -e in the beginning of the script, to help catch errors early if they happen (a best practice for bash scripts).

Note: excess whitespace was automatically removed by my text editor.

Test Plan

• Run the script to download the dataset.