Fallback to reading streamer and raise better error messages on true failures.

[jpivarski]

• Reproduce the failure in demo-double32.root without trying to read Double32. (Just load TTree metadata.)
• Add breadcrumbs to know where we are in the deserialization process. (Perhaps in context.)
• Create a new exception for deserialization failures and include the breadcrumbs in the message.
• Ensure that it's possible to locally override the classes dict.
• Catch that new exception in TKey.get and fallback to reading the streamer before actually giving up.
• The demo-double32.root example should then work. That's the end: close PR and move on!