minor: handle unchecked exceptions when opening inputstream

SVN: 21517

datastore_server/source/java/ch/systemsx/cisd/openbis/dss/generic/server/ftp/resolver/FtpFileImpl.java

@@ -82,7 +82,11 @@ public class FtpFileImpl extends AbstractFtpFile
         } catch (IOException ioex)
         {
             content.close();
-            return null;
+            throw ioex;
+        } catch (RuntimeException re)
+        {
+            content.close();
+            throw re;
         }
     }