pFq: prevent spurious overflow.

[jzmaddock]

Make sure we throw the correct exceptions when the result is infinite. Fixes #1404.

[codecov]

Codecov Report

❌ Patch coverage is 94.73684% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.39%. Comparing base (9a964b4) to head (31b1bc1).

Files with missing lines	Patch %	Lines
...tions/detail/hypergeometric_pFq_checked_series.hpp	88.23%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1406      +/-   ##
===========================================
- Coverage    95.39%   95.39%   -0.01%     
===========================================
  Files          826      826              
  Lines        68901    68925      +24     
===========================================
+ Hits         65729    65750      +21     
- Misses        3172     3175       +3     

Files with missing lines	Coverage Δ
...oost/math/special_functions/hypergeometric_pFq.hpp	100.00% <100.00%> (ø)
test/test_pFq.hpp	89.21% <100.00%> (+1.43%)	⬆️
...tions/detail/hypergeometric_pFq_checked_series.hpp	88.75% <88.23%> (-0.45%)	⬇️

... and 1 file with indirect coverage changes

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