Add new command epc:pop-to-last-server-process-buffer

When a server process failed to start process, it would be nice
if there is a way to see its error message.  This patch adds
a simple command to do that.
Mention the debugging command in the error message
Merge pull request #8 from tkf/pop-to-last-server-process-buffer

Add new command epc:pop-to-last-server-process-buffer