More robust handling of invalid config

[jieyouxu]

If we fail to deserialize config or whatever, we want to fail hard but with better diagnostics:

• Report where we tried looking for a valid config
• Report which config was attempted to be deserialized and failed
• Provide a suggestion (backup + nuke) the problematic config

In particular the configs being generation by the same version of mint does not guarantee it can be successfully deserialized, given that it's possible for the configs to be corrupted for reasons such as:

• Incorrect manual manipulation
• Partially written config
• Hardware failures
• Radiation-induced bit flipping

[jieyouxu]

Hmm this might also have to do with how we do the config serialization logic. The current temp-file-and-rename logic is atomic on Linux, but is not guaranteed atomic on Windows in presence of an existing config file.