Simplify open_socket by raising errors

This commit updates open_socket to raise errors, and return nil when it
succeeds. This simplifies both the method, and usage when implemented.
Without this change the intent is less clear.

lib/vmpooler/pool_manager.rb

@@ -35,11 +35,8 @@ def open_socket(host, domain=nil, timeout=5, port=22, &block)
           yield sock if block_given?
         ensure
           sock.close
-          return true
         end
       end
-    rescue SocketError
-    rescue Timeout::Error
     end
 
     def _check_pending_vm(vm, pool, timeout, vsphere)
@@ -49,8 +46,7 @@ def _check_pending_vm(vm, pool, timeout, vsphere)
         fail_pending_vm(vm, pool, timeout, false)
         return
       end
-      socket = open_socket vm
-      fail_pending_vm(vm, pool, timeout) if ! socket
+      open_socket vm
       move_pending_vm_to_ready(vm, pool, host)
     rescue
       fail_pending_vm(vm, pool, timeout)
@@ -145,8 +141,9 @@ def check_ready_vm(vm, pool, ttl, vsphere)
             $logger.log('s', "[!] [#{pool}] '#{vm}' not found in vCenter inventory, removed from 'ready' queue")
           end
 
-          socket = open_socket vm
-          if ! socket
+          begin
+            open_socket vm
+          rescue
             if $redis.smove('vmpooler__ready__' + pool, 'vmpooler__completed__' + pool, vm)
               $logger.log('d', "[!] [#{pool}] '#{vm}' is unreachable, removed from 'ready' queue")
             else