Improve error reporting for failing input files

[fedorov]

In the instances where input DICOM files have issues, it may be difficult to debug it since the converter does not provide specifics on which file caused the trouble. The function where those errors happen, unfortunately, does not know about the mapping from the DcmDatasets to the input file name ...

https://github.com/QIICR/dcmqi/blob/master/include/dcmqi/ConverterBase.h#L279