pdl: Propagate parse errors to callers

The generated test files now have 174 explicit unwraps.

Tag: #feature
Bug: 279758009
Test: atest pdl_tests pdl_rust_generator_tests_{le,be} pdl_generated_files_compile
(cherry picked from https://android-review.googlesource.com/q/commit:e45178e30b17b88aa69accb6ea964b9ac6d6cbc8)
Merged-In: Icd1819fb462df054590bbdeeb58143664b5cec6d
Change-Id: Icd1819fb462df054590bbdeeb58143664b5cec6d