Provide underlying reason for inability to load configuration files

[fbrier-beckman]

Type of Issue
[ ] Bug [X] Enhancement [ ] Compliance [ ] Question [ ] Help wanted

Describe the Issue

A variety of problems result in the message, "Could not load configuration file" It could be a missing file, but it could also be a bad configuration. In one case, the default namespace was not configured. I would be nice if the underlying error message was returned even nested in an inner exception.

To Reproduce
Steps to reproduce the behavior:

1. Remove the default namespace from the XML configuration file.
2. The exception merely says, "Could not load configuration file".

Expected behavior
A message such as "Unexpected element ApplicationConfiguration found on line 2".

Environment (please complete the following information):

• OS: Windows 10
• Development environment: Visual Studio 2019 16.7.7
• Runtime: .NET Core 3.1
• OPC Package Release Version [e.g. 1.4.363.107]
• Component: [e.g. Opc.Ua.Core]
• Server: Custom Server
• Client: C# - custom