make develop_server.sh notify/cleanup when pelican or server don't start

After waiting for pelican and server to come up, if either one has
died then give a more helpful message and clean up. Previously did not
check for this, so script informed user that everything was running even
if one or both parts failed for whatever reason.

This is meant to provide a little more user-friendliness
in those cases where user has a develop_server.sh in project directory
but forgot to (re)install pelican, activate relevant virtualenv, etc.
as well as other unforeseen situations where one of the processes does
not start.