Updated to 5.4, can no longer build and run.

Since I didn't get any response back regarding my networking question, I resolved to update Unity in order to follow a tutorial that uses features not found in 5.2.3, in the hope a different method will solve my issue.

After the API update, I was met with errors. So, I've deleted or commented out everything that I am/was using that has to be replaced later. It works fine if I press play, though although of course it's missing certain features.

Now, if I try to build and run, it spits out these errors: http://i.imgur.com/UgJLVAD.png

If I press play, even afterwards, the errors clear and I can play in the editor just fine.

I tried to troubleshoot my way through this by simply deleting the scripts (and the entire standard assets folder) Unity is complaining about since they're just unused assets. Unfortunately every time an error goes away, another bunch appear. It's quite happy to spit out 45 errors referencing scripts that aren't even there anymore!

Can somebody tell me how to sort this?