Use concrete built-in exceptions instead of Exception base class

[hugovk]

Fixes #168.

Instead of using the Exception base class, use concrete built-in IndexError, TypeError and ValueError instead.

https://docs.python.org/3/library/exceptions.html

Allows more targeted exception catching.

[codecov]

Codecov Report

Merging #169 (d1d9d55) into master (f7c1f43) will not change coverage.
The diff coverage is 39.39%.

@@           Coverage Diff           @@
##           master     #169   +/-   ##
=======================================
  Coverage   93.42%   93.42%           
=======================================
  Files           5        5           
  Lines        2130     2130           
=======================================
  Hits         1990     1990           
  Misses        140      140           

Flag	Coverage Δ
GHA_Ubuntu	93.42% <39.39%> (ø)
GHA_Windows	93.28% <39.39%> (ø)
GHA_macOS	93.42% <39.39%> (ø)

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Impacted Files	Coverage Δ
src/prettytable/prettytable.py	89.48% <23.07%> (ø)
tests/test_prettytable.py	100.00% <100.00%> (ø)

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[fawaf]