When relaunching: only close program if recompile is successful

[reinerp]

See http://code.google.com/p/yi-editor/issues/detail?id=355 for the relevant ticket for Yi.

Basically, I would like better control over recompilation. Specifically:

1. When using the functions in Config.Dyre.Relaunch I don't want to shut down the UI until I know that recompilation has succeeded. If recompilation has failed, I want to abort the relaunch and display the error messages to the user.
2. (Less important, but still nice.) I would like to be able to recompile the config file without triggering a program relaunch. This is useful in Yi if I want to check that my config file compiles, but I don't want to actually use it (yet). This can mostly be implemented using Config.Dyre.Compile.customCompile, but this function will always trigger a recompile, whereas I only want to recompile when my config file has changed.