Export Instruments: change back to original dir at end

[joshbarrass]

Currently export_instruments will create a new dir, cd to it, then export, but will not go back to the original dir afterwards. If running this function multiple times, this can create a set of messy, nested directories. This changes back to the original dir when exporting is finished.

Also introduces more specific handling of FileExistsError when attempting to create the new directory, rather than blindly catching all errors.