If the installer only proceeded to the point where it cannot download the actual /e/ OS files, nothing much has changed on your phone, so the stock Android by Fairphone is still there, i.e. no need to reinstall it.
At that point, the only change is that the bootloader was unlocked (which should give you a warning when turning on the phone) which also wipes the data.
In that case you only need to re-lock the bootloader and the phone should just boot up normally and present you with the first time setup wizard.
But to confirm my assumption: can you try to boot your phone just as if nothing had happened? It should come up after a warning where you need to confirm to continue (or just wait a few second).