[Datumaro] Add debug output in VOC import

[zhiltsov-max]

Motivation and context

Some debug output should help in understanding dataset import status and the source of errors.

How has this been tested?

Manual test

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG file
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (read github docs)
• I have increased versions of npm packages if it is necessary (cvat-canvas,
  cvat-core, cvat-data and cvat-ui)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2020 Intel Corporation
#
# SPDX-License-Identifier: MIT

[nmanovic]

@zhiltsov-max , Need to fix tests. It seems that the code doesn't work.

[coveralls]

Pull Request Test Coverage Report for Build 5543

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• 14 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.2%) to 65.833%

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Files with Coverage Reduction	New Missed Lines	%
cvat/apps/engine/media_extractors.py	1	77.35%
datumaro/datumaro/components/extractor.py	6	91.01%
datumaro/datumaro/plugins/datumaro_format/converter.py	7	95.37%

Totals
Change from base Build 5516:	0.2%
Covered Lines:	10919
Relevant Lines:	16192

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💛 - Coveralls