1432533 - Avoid file.exists() --> read file antipattern in migrator code

Status: NEW

(Firefox :: Migration, enhancement, P3)

Description

[Alex Thayer [:alexical] (she/her)]

Created as a follow-up for Bug 862127.

There's a number of places in out migrator code where we check if files exists before reading them, when the faster thing to do would be to just attempt to read them and throw if we can't. In some cases, we're using the file.exists check to determine whether to show UI or not, so we'll need to split out the UI checks from the program logic checks.