RBUTILS-027: Improve logged info by the plain text logger

Summary: Improve logged information by the plain text logger.
We just need to display:
- the name of the task
- the path to the root directory
- only errors: relative path to the file which has error, type of error and message

This must be done for both command line and ant task.

Work to do:
- This currently seems to be ok with the ant task !!!
- Remove messages of "file started" and "file ended" which are messy; we will maybe display them when we will have categories for messages and provide an INFO state
- RbChecker: plain logger: must display RbChecker instead of null
- RbChecker: fix the {1} message problem; when missing key, this is not replaced by real value. This is not needed: replaced with "No found key in the ResourceBundle"
- RbChecker: make log messages clearer