Add a flag to show reloading info only in case of problems

[bbatsov]

One user complained on gitter that he finds the window useless most of the time. One idea would be to add a flag to show it only when there are some issues. I'm open to other suggestions. At any rate - the default will continue to be the current behaviour.

[cichli]

If there are issues, the error buffer will popup with the stacktrace anyway. I think an option to not show the log buffer (it will still be logged to), and instead display the most important messages (reloading started/successful/failed, calling/called before/after fns) in the echo area will do the trick.

[bbatsov]

Sounds good to me.