manifold_point_error can't return multiple independent issues

[mateuszbaran]

As discussed in #46 , the current approach to error checking allows reporting only a single error at a time. For some manifolds (for example product and power manifolds) multiple independent issues may arise at the same time. We should have a way to report all of them in manifold_point_error.

Ref #24 .

[kellertuer]

I think I found a good solution, for power and product, within check_manifold_point or check_tangent_vector do

1. call all checks, collect results in a vector,
2. if all entries are nothing (maybe Filter out nothings and check whether empty)
3. if the result of 2 is not empty, wrap that in a CompositeException https://docs.julialang.org/en/v1/base/base/#Base.CompositeException
   return that.

[mateuszbaran]

That is a good solution.