More user-friendly exceptions

[smarie]

Fixes #5

[coveralls]

Coverage increased (+0.2%) to 96.825% when pulling 26028a8 on smarie:fix_issue_5 into 66c71a7 on tqdm:master.

[casperdcl]

including six just to suppress a bit of trace? okay...

[smarie]

Well in my experience six eventually pops up sooner or later when you try to cover such a range of python versions :)

I eventually used it in most of my libraries, nobody complained about it. Note however that I never had to add it as an explicit dependency in setup.py, and the builds were ok under travis.
This is maybe related with the fact that tox environments are more raw than travis default environments..

[codecov-io]

Codecov Report

Merging #7 into master will increase coverage by 0.42%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master       #7      +/-   ##
==========================================
+ Coverage   94.06%   94.48%   +0.42%     
==========================================
  Files           6        6              
  Lines         118      127       +9     
  Branches       25       25              
==========================================
+ Hits          111      120       +9     
  Misses          4        4              
  Partials        3        3

[smarie]

I just pushed an update with coverage correction. The failing travis build seems to be related to the fact that python 2.6 is not available in travis anymore..

[smarie]

https://travis-ci.community/t/issue-with-python-2-6-on-linux/3861