Getting the same error on my Lumia 521.
Easiest way is probably to install the Windows Phone Recovery Tool.
https://www.windowsphone.com/en-US/how-to/wp8/phones-and-hardware/recover-my-phone
***BACKUP FIRST (if you didn't back up just before you first attempted to install new build)
You can also go to Settings, System, About, and use the Reset Phone function. That will bring you back to factory settings-- but it wipes out all of your apps, settings, etc. so you'll want to back up beforehand and then use the Restore option during the set up process.
Gotta say, though, I'm getting pretty frustrated with the insider preview for W10M. I know they aren't finished yet, and they warn upfront that you shouldn't install it on your main/ daily driver phone, but it just seems like they should be further along at this point. I keep thinking "Oh, this next build will be the one where they get it together", but the time interval between builds and the errors like this one are turning me off.