Throw MethodError instead of StackOverflowError for gamma, loggamma, and logabsgamma

[devmotion]

This PR ensures that MethodErrors are thrown instead of StackOverflowErrors if gamma, loggamma and logabsgamma are called with arguments for which no fallback exists. Additionally, the dispatches for gamma and logabsgamma are simplified.

It is a more general alternative to #295.

Fixes #339.
Fixes #233.

[codecov]

Codecov Report

Merging #347 (6f203cf) into master (1b7a377) will increase coverage by 0.67%.
The diff coverage is 95.77%.

@@            Coverage Diff             @@
##           master     #347      +/-   ##
==========================================
+ Coverage   92.23%   92.90%   +0.67%     
==========================================
  Files          12       12              
  Lines        2717     2706      -11     
==========================================
+ Hits         2506     2514       +8     
+ Misses        211      192      -19     

Flag	Coverage Δ
unittests	92.90% <95.77%> (+0.67%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/SpecialFunctions.jl	100.00% <ø> (ø)
src/gamma.jl	94.82% <95.77%> (+0.26%)	⬆️
src/chainrules.jl	100.00% <0.00%> (ø)
src/gamma_inc.jl	92.79% <0.00%> (+0.16%)	⬆️
src/beta_inc.jl	91.27% <0.00%> (+2.40%)	⬆️

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[stevengj]

Thanks!