Proper errors on Power and Product

[kellertuer]

To continue #207 this PR introduces two special error types for composite manifolds like power and product: the ComponentManifoldError consists of an error and an index, where this error occurred,
the CompositeManifoldError collects a set of these if there is more than one error.

Maybe a separate docs page would be still nice.

[codecov]

Codecov Report

Merging #210 into master will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #210      +/-   ##
==========================================
+ Coverage   95.03%   95.12%   +0.09%     
==========================================
  Files          63       64       +1     
  Lines        3744     3774      +30     
==========================================
+ Hits         3558     3590      +32     
+ Misses        186      184       -2     

Impacted Files	Coverage Δ
src/Manifolds.jl	100.00% <ø> (ø)
src/errors.jl	100.00% <100.00%> (ø)
src/manifolds/MultinomialDoublyStochastic.jl	100.00% <100.00%> (ø)
src/manifolds/PowerManifold.jl	95.91% <100.00%> (+0.04%)	⬆️
src/manifolds/ProductManifold.jl	89.65% <100.00%> (+0.08%)	⬆️
src/manifolds/Euclidean.jl	91.86% <0.00%> (+0.09%)	⬆️
src/groups/group.jl	77.77% <0.00%> (+1.05%)	⬆️

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