Enhance the error message for datum stats to be more user friendly

[vinnamkim]

Summary

• Ticket no. 112203
• Improve the error message for users encountered in the following CLI use case:

(datumaro) vinnamki@vinnamki:~/datumaro/ws_datum/stats$ datum project create
(datumaro) vinnamki@vinnamki:~/datumaro/ws_datum/stats$ datum project import -n eurosat -f imagenet_txt /home/vinnamki/datumaro/tfdseurosat-imagenet_txt.1
(datumaro) vinnamki@vinnamki:~/datumaro/ws_datum/stats$ datum project import -n uc_merced -f imagenet_txt /home/vinnamki/datumaro/tfdsuc_merced-imagenet_txt

(Before)

(datumaro) vinnamki@vinnamki:~/datumaro/ws_datum/stats$ datum stats
2023-06-28 17:09:27,574 ERROR: Failed to parse revspec:
  Merging of datasets with different categories is only allowed in 'merge' command.
  Path project doesn't exist

(After)

(datumaro) vinnamki@vinnamki:~/datumaro/ws_datum/stats$ datum stats
2023-06-28 17:55:20,580 ERROR: There are more than two sources with heterogeneous categories in the project. This prevents computing the statistics of the merged one. Please specify one of the sources in the project (['eurosat', 'uc_merced']), such as `datum stats eurosat`

How to test

Checklist

• I have added unit tests to cover my changes.​
• I have added integration tests to cover my changes.​
• I have added the description of my changes into CHANGELOG.​
• I have updated the documentation accordingly

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) 2023 Intel Corporation
#
# SPDX-License-Identifier: MIT

[sooahleex]

Looks good to me.

[wonjuleee]

LGTM