Print informative error messages if upload fails.

[csukuangfj]

It wastes me more than 3 hours to figure out the real reason why the upload fails. Hope this pull-request
can avoid wasting other's time.

The code is from #501 (comment)

It's also used by pytorch/audio#354 (comment)

• Closes anaconda upload silently fails #501
• Closes anaconda upload fails silently if the user's storage limit is exceeded #516
• Closes Anaconda doesn't failed on expired authentication token #536

[Viech]

Within a temporary conda environment, one can use the commit referenced in this PR to monkey-patch anaconda-client as follows:

# HACK: Patch anaconda-client to show an error message.
conda install wget git
cd "$(conda info|grep 'active env location'|awk '{print $5}')/lib/python"*"/site-packages/"
wget https://github.com/Anaconda-Platform/anaconda-client/commit/bb74857cbc0b59e9c328fbf7e31caaedbc1bb9f9.diff
git apply bb74857cbc0b59e9c328fbf7e31caaedbc1bb9f9.diff
cd -

This has worked for me using the conda/miniconda3 docker image. The environment has to be active and anaconda-client needs to be installed therein.