Ensure ProtoLogTool processing errors are reported

The ProtoLogTool was failing to report processing errors at the end of
execution.

The list used to store processing errors was being re-initialized, which
caused any errors caught during processing to be discarded instead of
being reported.

This change ensures that the same error list is used throughout the
tool's lifecycle, allowing processing errors to be correctly collected
and reported.

Bug: 370425985
Flag: EXEMPT build tool changes only
Change-Id: Ica2eadb4cc612d6b860a7aaf3df7b5521567e492