[Feature Request] More clear error message in case of problem with relative path

[denisbredikhin]

Hello,

in one of my md files I had the wrong path to another article (something like "~/docs/frameworks/api/components/httpclient.md"). When I was trying to build my documentation, I was getting exception: System.NotSupportedException: Relative to path has too many '../'. Which didn't tell anything about the file or path that has this problem.

I've built the local version of RelativePath class and changed the following line in the method MakeRelativeTo: throw new NotSupportedException("Relative to path has too many '../'");

to

throw new NotSupportedException($"Relative to path ({relativeTo}) has more '../' than base path {this}.");

Now I can see which path has the problem. Can you please include this small improvement to docfx?