More informative error messages

[palday]

• More informative error messages when passing a Distribution or Link type instead of the desired instance.
• More informative error message on the intentional decision not to define methods for the coefficient of determination.
• NB: we still don't export r2 or adjr2 but these functions might be scope from some other package. We just define methods for them.

Not sure how I feel about adding in these safety features -- @dmbates please feel free to reject all or part of this.

inspired by #696 and #697

• add entry in NEWS.md
• after opening this PR, add a reference and run docs/NEWS-update.jl to update the cross-references.
• I've bumped the version appropriately

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.10% 🎉

Comparison is base (5ade8c4) 95.72% compared to head (bf2b1ec) 95.82%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #698      +/-   ##
==========================================
+ Coverage   95.72%   95.82%   +0.10%     
==========================================
  Files          35       35              
  Lines        3226     3259      +33     
==========================================
+ Hits         3088     3123      +35     
+ Misses        138      136       -2     

Flag	Coverage Δ
current	95.82% <100.00%> (+0.10%)	⬆️
minimum	95.72% <100.00%> (+0.10%)	⬆️
nightly	95.82% <100.00%> (+0.10%)	⬆️

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

Files Changed	Coverage Δ
src/generalizedlinearmixedmodel.jl	90.36% <100.00%> (+0.11%)	⬆️
src/linalg/rankUpdate.jl	98.30% <100.00%> (+0.10%)	⬆️
src/linearmixedmodel.jl	94.97% <100.00%> (+0.09%)	⬆️
src/mixedmodel.jl	92.53% <100.00%> (+1.30%)	⬆️
src/remat.jl	97.17% <100.00%> (+0.47%)	⬆️

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