Improve error messages

[ewjoachim]

Checklist:

• Tests
  • (not applicable?)
• Documentation
  • (not applicable?)

[codecov]

Codecov Report

Merging #23 (64e60d4) into master (7a22d34) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #23   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines          135       140    +5     
  Branches        10        11    +1     
=========================================
+ Hits           135       140    +5     

Impacted Files	Coverage Δ
pypitoken/__init__.py	100.00% <100.00%> (ø)
pypitoken/exceptions.py	100.00% <100.00%> (ø)
pypitoken/token.py	100.00% <100.00%> (ø)

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