Fix error type for external errors during serialization

[fxamacker]

The wrong error type was returned when an external error is encountered (in atree inlining feature branch).

This PR returns the correct error type.

NOTE: This problem does not exist in the master branch.

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[fxamacker]

@turbolent @ramtinms PTAL :pray: This needs to be included as part of using atree inlining in Cadence.