Better error handling through improved Display and Error impls

[H2CO3]

What does this PR do?

This improves user-facing impls (primarily related to error reporting) such as Display and Error, in the following ways:

• manual Display impls are provided when user-visible descriptions are needed, and {} formatting is used instead of {:? where appropriate. Debug is not suitable for this purpose, as its output is neither human-readable nor stable nor reliable.
• In particular, Debug formatting may very well print absolutely NOTHING at all, so it's basically completely unreliable, and that is on purpose. See e.g. debug-fmt-detail option rust-lang/rust#123940 for details.
• Error impls are provided for all error-like types where missing, and the source() method is implemented in a more useful manner where possible, so as to provide error chaining/causality information.
• Underlying errors are stored in error types when appropriate.
• Paths are displayed as intended, i.e., using the .display() method instead of Debug-formatting.
• The Display impl for the Disp type is simplified.
• Some unnecessary allocations of temporary strings (.to_string() and format!()) are eliminated.

The Python bindings are also updated according to these principles. The Python API is not affected.