Raise on fatal errors instead of exit

So that the errors can be caught in specs.
In lavinmq.cr is the exit done

src/lavinmq.cr

@@ -17,8 +17,12 @@ config.parse # both ARGV and config file
 require "./lavinmq/launcher"
 require "./lavinmq/clustering/controller"
 
-if config.clustering?
-  LavinMQ::Clustering::Controller.new(config).run
-else
-  LavinMQ::Launcher.new(config).run
+begin
+  if config.clustering?
+    LavinMQ::Clustering::Controller.new(config).run
+  else
+    LavinMQ::Launcher.new(config).run
+  end
+rescue LavinMQ::Etcd::NoEtcdEndpoint
+  exit 5 # 5th character in the alphabet is E(etcd)
 end

src/lavinmq/etcd.cr

@@ -258,6 +258,8 @@ module LavinMQ
       end
     end
 
+    class NoEtcdEndpoint < Exception; end
+
     private def connect : Tuple(TCPSocket, String)
       @endpoints.shuffle!.each do |address|
         host, port = address.split(':', 2)
@@ -276,7 +278,7 @@ module LavinMQ
         next
       end
       Log.fatal { "No etcd endpoint responded" }
-      exit 5 # 5th character in the alphabet is E(etcd)
+      raise NoEtcdEndpoint.new
     end
 
     private def update_endpoints(tcp, address)