sheet2pcp: improve error handling and diagnostics a little
sheet2pcp: improve error handling and diagnostics a little

Verify the datetime further and list supported file extensions.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,wuliming/pcp,wuliming/pcp,adfernandes/pcp,wuliming/pcp,adfernandes/pcp,adfernandes/pcp,wuliming/pcp,wuliming/pcp,wuliming/pcp,wuliming/pcp,wuliming/pcp,adfernandes/pcp,adfernandes/pcp