Add label check on import

[zhiltsov-max]

Motivation and context

• Added more informative error output for annotation import errors (image name, annotation index, annotation type)
• Added a check for empty label

How has this been tested?

Manually

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) 2021 Intel Corporation
#
# SPDX-License-Identifier: MIT

[coveralls]

Coverage increased (+0.02%) to 72.235% when pulling 8d81cd1 on zm/add-no-label-import-check into 9f7d181 on develop.

[nmanovic]

If it isn't the only way to raise the exception inside try/except block, probably you want to remove try/except in the case.

[zhiltsov-max]

It isn't, but the idea is to provide an informative wrapper for better investigation experience.