Correct error handling in mpfr specfun overloads.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.1%. Comparing base (42a3edf) to head (ab15f2f).

:exclamation: Current head ab15f2f differs from pull request most recent head fe60def. Consider uploading reports for the commit fe60def to get more accurate results

Additional details and impacted files

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[jzmaddock]

Last failure is a network timeout on drone.

@mborland with the changes in Math develop, this now gets Multiprecision develop green and good to go. Math will need a second merge to master before Multiprecision.