Support setting non-standard isolation level

For JDBC drivers (like Informix) isolation level 5 is a valid value. Given JDBC Connection.transactionIsolation is just an int, it doesn't make sense to limit the possible values to what is defined in JDBC Connection class

Author: ianbondoc

ebean-datasource/src/main/java/io/ebean/datasource/pool/TransactionIsolation.java

@@ -25,7 +25,7 @@ static String getDescription(int level) {
       case -1:
         return "NotSet";
       default:
-        throw new RuntimeException("Transaction Isolation level [" + level + "] is not defined.");
+        return "UNKNOWN[" + level + "]";
     }
   }
 }