Add a better notification when you need to accept Apple's terms on the website

[Sjmarf]

When I tried to download Xcode, I got the error "Unable to install Xcode; if you continue to have problems, please submit a bug report in the help menu". It turned out that I needed to sign into the Apple Developer website and accept their terms, and then I was able to download it.

It would be nice if Xcodes detected this error case and gave a more informative error message, or if it's not possible to detect, it could list that as a possible fix.

I recently changed my Apple ID password, which may be why I was required to accept the terms again.