document function `rebuild_cmdstan`; add detection of need to rebuild gch headers; improve error message

[mitzimorris]

Summary:

When compilation fails because the user needs to rebuild cmdstan, detect this failure and make a better error message.

Add troubleshooting section to the docs, and document function rebuild_cmdstan.

Description:

This is very frustrating for MacOS users, because when Xcode changes, CmdStan needs to be rebuilt.