fix: exception message assembling

fix: exception message assembling: a task in bird-critic-1.5-full-300 (Harbor dataset). API exceptions raised by this SDK currently produce messages that are hard to act on: the PayPal error code, the human-readable reason and the per-field validation details are not assembled into one coherent…

The task

API exceptions raised by this SDK currently produce messages that are hard to act on: the PayPal error code, the human-readable reason and the per-field validation details are not assembled into one coherent sentence. Fix the message assembly so an exception reads as a complete diagnosis.

Part of kochsnow/bird-critic-1.5-full-300.