The final fallback in seasonConversion blindly constructed a filename
without checking if it existed on disk. When the file was missing,
handleMsgSysGetFile would send doAckBufFail, but the original Frontier
client does not gracefully handle this during quest loading — causing a
softlock instead of showing the built-in error dialog.
Now every fallback path validates file existence before returning, and
also tries the opposite time-of-day variant as a last resort. If no
file variant exists at all, the original filename is returned with a
warning log so the failure ack is still sent.
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