fix: Allow choice fallback after traversal errors

[mvanhorn]

Update traversal error generation in NimbleParsec.Compiler to honor the active catch_all continuation already carried in compiler configuration, following the failure-routing pattern used by other unbound combinators. post_traverse/3 callbacks may return {:error, reason}, but the compiler currently turns that result directly into the parsec's final error tuple.

A choice/2 whose first branch consumes input and then succeeds in a post-traversal callback returns that branch's transformed result without evaluating the fallback behavior; A post-traversal callback that returns {:error, reason} causes choice/2 to retry a later branch from the original input, producing the fallback result with correct rest, context, position, and byte offset.

Closes #146

[josevalim]

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