browser.toJSON(). Handle attempt to add local files. (#1162)
browser.toJSON(). Handle attempt to add local files. (#1162)

* browser.toJSON(). Handle attempt to add local files.

* Reword error messages for local files in sessions

Co-authored-by: Jim Robinson <1cfd0d483c5d5894a9e313c6bb220d638aaa9894@broadinstitute.org>
igvteam/igv.js,igvteam/igv.js