Got an "Oops" Screen While in Installer, Exited to Try Again, and Now I Can't Get Back to Installer
I feel silly posting an answer to my own question, but I wanted to post the solution in case anyone else ran across this, instead of just deleting the question.

I was using the URL http: //localhost/public/index.php/admin/install to get to the installer, and I think it wouldn't let me go back because the installation-- somehow-- was actually successful. I started playing around with the URL and found that http: //localhost/public/index.php/admin brought me to the admin dashboard. Trying to put in the address I gave the site is still throwing a Page Not Found at me, but I'm hoping that's because everything is there but not quite wired-up correctly yet; I'll try to come back and edit this if I find that there is more that should be included here to remedy this.