I was editing using MAMP as I don’t have a server yet.
MAMP is a server– well, server package. The “A” stands for “Apache”, as in “Apache web server”.
Is there anything I can do?
Edit the files with a real code editor instead of that travesty of an editor built into WordPress. On a Mac, Textwrangler is solid as is its big brother BBedit. Jedit is pretty good too.
Grab an editor and brows to your server’s web root. I don’t remember the defaults for MAMP but it ought to have a help file. Edit away.
You will need to configure your server to show errors. Also, you probably want to turn on WordPress’ debugging while you work. With that done, you should see messages telling you the file name and the line number of the error.