Do not thrown an exception if the network pool is empty; let the re-fill

When remote server goes away and comes back, the networkPool is empty, but the checkConnection() method prevents reattempting a connection to the remote database.  See discussion at https://groups.google.com/forum/#!topic/orient-database/ePkpTaj-gxk.  Similar code exists in develop branch and 1.6.0-SNAPSHOT.  Consider backporting to 1.5.x hotfix.