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

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?