Validate config file encoding when it contains non-ascii characters

Currently if a config file contains non-ascii characters but is not "properly" encoded, one would get the unexpected error:
    
    invalid byte sequence in UTF-8

This fix propose to check if the config file is correctly encoded otherwise throw a more verbose error message + hint to convert the failing file(s) to UTF-8

Related tickets:
https://logstash.jira.com/browse/LOGSTASH-1103
https://logstash.jira.com/browse/LOGSTASH-1531