ENYO-3060: show an error message to indicate to choose a file system if it's not the case

Related JIRA: https://enyojs.atlassian.net/browse/ENYO-3060

Add an error message in HermesFileTree to indicate if no FileSystem has been selected.

Enyo-DCO-1.1-Signed-off-by: Vincent HERILIER vincent.herilier@hp.com