Resolve Deprecated warnings in the Error traits

[dsietz]

Resolve the Deprecated warnings in the following traits:

1. pbd::dpi::error::Error
2. pbd::dsg::error::Error
3. pbd::dtc::error::Error
4. pbd::dua::error::Error

The impacted methods are the following:

• description(&self) -> &str
  Deprecated since 1.42.0: use the Display impl or to_string()
• cause(&self) -> Option<&dyn Error>
  Deprecated since 1.33.0: replaced by Error::source, which can support downcasting

[dsietz]

The Rust lang std::io::Error also has these warnings.