Minor fixes

[nolze]

• Improve error message
• Update README
• Improve version handling

[codecov-io]

Codecov Report

Merging #20 into master will increase coverage by 0.1%.
The diff coverage is 5.88%.

@@            Coverage Diff            @@
##           master      #20     +/-   ##
=========================================
+ Coverage   38.44%   38.55%   +0.1%     
=========================================
  Files          12       12             
  Lines         770      773      +3     
=========================================
+ Hits          296      298      +2     
- Misses        474      475      +1

Impacted Files	Coverage Δ
msoffcrypto/format/xls97.py	39.37% <0%> (ø)	⬆️
msoffcrypto/format/doc97.py	31.96% <0%> (ø)	⬆️
msoffcrypto/__main__.py	0% <0%> (ø)	⬆️
msoffcrypto/format/ooxml.py	51.06% <0%> (+1.06%)	⬆️
msoffcrypto/__init__.py	50% <50%> (+3.84%)	⬆️

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