provide less fear-inducing messages when importing

[alecristia]

I imported alice & vtc a moment ago, and got a lot of:
09/13/2023 19:55:27 : ERROR : Error: annotation file annotations/alice/output/converted/3249a_0_10753816.csv already exists, to reimport it, use the overwrite_existing flag

I thought that meant the whole importation had failed.

It may have been more informative to state "17 files were correctly imported. XX others were not, with the following error messages:"
and then the error messages