Use error messages in TaxCalcIO instead of Python "raise Error"

[martinholmer]

This pull request converts Python-style "raise Error" messages into simple error messages, more than one of which can be written to the screen via stderr. The reason for doing this is to further the goal of the CLI to Tax-Calculator, which is to make useful to those who know nothing about Python. The length "raise Error" traceback information is helpful only to Python programmers. Also, the "raise Error" approach to error handling stops program execute as soon the first error is found, while the approach adopted here can show several errors before stopping execution. This means users are likely to specify correct input more quickly than if errors are reported one at a time.

There is no change in substance in this pull request.

[codecov-io]

Codecov Report

Merging #1260 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1260      +/-   ##
==========================================
+ Coverage   99.67%   99.67%   +<.01%     
==========================================
  Files          38       38              
  Lines        2783     2805      +22     
==========================================
+ Hits         2774     2796      +22     
  Misses          9        9

Impacted Files	Coverage Δ
taxcalc/taxcalcio.py	100% <100%> (ø)	⬆️

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