pdl: Propagate child errors from Packet::new

The generated test files now have 106 explicit unwraps.

Tag: #feature
Bug: 279758009
Test: atest pdl_tests pdl_rust_generator_tests_{le,be} pdl_generated_files_compile
Change-Id: Ife894409a9c2e5f508ba291e4287bf6dfb69c293