Fix missing extension from message file causing panic

This pull request fixes panic caused by missing extension from i18n message file.

Steps to reproduce:
 1. Checkout: https://github.com/WinWiny/revel-i18n-bug
 2. $ revel run github.com/WinWiny/revel-i18n-bug
 3. Visit http://localhost:9000/
 4. Revel panics

This patch kindly instructs users of this error using log message.