improve handling of unsuccessfully content extraction

[jerrygaoLondon]

In current version (2.0-beta.1) of JATE2.0, the app will complain "uk.ac.shef.dcs.jate.JATEException: Cannot find expected field: jate_ngraminfo" if data format is not supported or simply no content can be extracted.

It is very confusing log information and not very indicative. We should log useful information to indicate if no content can be extracted and explain the cause if "cannot find expected field "