GUI-800 added warning for filereader support if missing

added warning for filereader support if missing


https://eucalyptus.atlassian.net/browse/GUI-800
@dkavanagh: Correct, we are using (and detecting) the FileReader interface from the File API, so we should explicitly state that our check tests for the FileReader API.  Stating that the browser doesn't support the File API when we are only checking for the FileReader API is misleading IMO.