Downloading issue with safari
It is sometimes possible to resume downloads in Safari. Click the download icon in the toolbar, and check the color of the "↻" icon. If the icon is gray, just click it to resume the download. However, if the icon is orange, do not click it, as Safari will delete the partially downloaded data and restart the download from scratch.

When Safari cannot resume a download, it is usually possible to resume it with curl instead. I do this so often, I actually made an Automator workflow to expedite the process.

https://github.com/Wowfunhappy/Automator-Quick-Actions/blob/master/Resume%20Download.workflow.zip?raw=true


Extract the zip file
Double click the .workflow and install it.


The next time a Safari download fails in the middle:


Go to the recent download list in Safari. If applicable, click the "x" to fully cancel the download, and then click the magnifying glass to reveal it in Finder. DO NOT click the orange retry ("↻") button; doing so will delete all partially downloaded data.
In Finder, right click on the partially downloaded file and select either Resume Download or Services → Resume Download.
Wait for the percentage in the terminal window to reach 100%. Note that this percentage does not include the part of the file that was downloaded previously, so it will always start at 0. If the download fails again, resume it again by focusing on the terminal window, hitting the up arrow key, and then the enter key.


Not all downloads can be resumed this way—some servers make it technically impossible—but in my experience it usually works.