No safety checks for file overwriting

[scgbckbone]

• for instance in wallet.py Wallet.export_menu (but with basic full text search I see it is on many places where files get written to SD)
• if file A.json already exists on SD card and my wallet has name A and I try to export it's descriptor A.json gets rewritten

from Wallet.export_menu

                fname = "/sd/%s.json" % self.name
                with open(platform.fpath(fname), "w") as f:
                    json.dump(obj, f)
                platform.unmount_sdcard()

Probably fail if file already exists in export location? Or at least ask user if he want' to rewrite?