Improve erf/expm1/expint coverage.

[jzmaddock]

In the expm1 case, tighten up error handling and testing.

[codecov]

Codecov Report

Attention: Patch coverage is 99.33775% with 1 line in your changes missing coverage. Please review.

Project coverage is 94.36%. Comparing base (5459e14) to head (ba6838c).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1111      +/-   ##
===========================================
+ Coverage    93.77%   94.36%   +0.58%     
===========================================
  Files          772      774       +2     
  Lines        61352    59754    -1598     
===========================================
- Hits         57535    56386    -1149     
+ Misses        3817     3368     -449     

Files	Coverage Δ
.../boost/math/special_functions/detail/bessel_ik.hpp	100.00% <100.00%> (ø)
.../boost/math/special_functions/detail/bessel_j1.hpp	100.00% <ø> (ø)
.../boost/math/special_functions/detail/bessel_k0.hpp	100.00% <ø> (ø)
.../boost/math/special_functions/detail/bessel_k1.hpp	100.00% <ø> (ø)
...ost/math/special_functions/detail/igamma_large.hpp	100.00% <ø> (+66.88%)	⬆️
...ost/math/special_functions/detail/lgamma_small.hpp	100.00% <100.00%> (+40.63%)	⬆️
.../boost/math/special_functions/detail/polygamma.hpp	91.38% <ø> (-0.64%)	⬇️
...h/special_functions/detail/unchecked_factorial.hpp	99.10% <ø> (-0.90%)	⬇️
include/boost/math/special_functions/erf.hpp	100.00% <ø> (+0.45%)	⬆️
include/boost/math/special_functions/expint.hpp	100.00% <ø> (+2.03%)	⬆️
... and 29 more

... and 8 files with indirect coverage changes

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