Add a base exception for all taopq exceptions.

[MeanSquaredError]

This PR adds a base_error exception type from which all the other exception types (timeout_reached, network_error, sql_error) derive. This allows catching all possible taopq exceptions with a single catch block. Earlier I suggested this change in #68.

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (fc080a6) 99.65% compared to head (40cef3e) 99.65%.
Report is 1 commits behind head on main.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main      #69   +/-   ##
=======================================
  Coverage   99.65%   99.65%           
=======================================
  Files          68       68           
  Lines        2297     2297           
=======================================
  Hits         2289     2289           
  Misses          8        8           

Files Changed	Coverage Δ
src/lib/pq/exception.cpp	100.00% <ø> (ø)

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