Simplify user exception mapping

[bernardnormier]

User exceptions are more like result failures (see https://docs.icerpc.dev/slice2/language-guide/result-types) than actual exceptions.

As a result, a mapped user-exception does not need:

• the ability to set a custom error message during construction
• the ability to set an inner exception

Furthermore, a client-side application does not need to ever create a user exception. So we don't need to provide/document a constructor/initializer for mapped user exceptions in client-only mappings (MATLAB, PHP, Ruby).

Fixed in:

• [x] C++
• [x] C#
• [ ] Java
• [x] JavaScript
• [x] MATLAB
• [x] PHP
• [ ] Python
• [x] Ruby
• [x] Swift