Better error message for saving a list with an existing name

[ValWood]

If you try t save a list with the same name as a previous list you get an error message "failed to save list" but you have no idea why. it would be useful if the message says
" al list already exists with this name"

[heralden]

Technical notes: im-tables-3 doesn't extract the error message from response body error: "Attempting to rename a bag to a new name that already exists: asdf" while the upload page does this.