Don't propagate unmount error on windows to UI if verification succeeded.

[zwhitchcox]

A lot of users are getting errors that the unmount operation failed:

balena-io/etcher#3542
balena-io/etcher#3730
balena-io/etcher#1370

...etc.

Even though the verification makes it to 100%.

We can see that even if a user has a task manager open, the device can be marked as busy:

balena-io/etcher#1316

Even though nothing has changed, the task manager just has the device open, so it is marked as "busy".

If we make it to 100% verification, the flashing has succeeded, and so there is no need to propagate a failure error to the user, because the user thinks that something has gone wrong with the flashing process.

We should only show the error if the verification has failed.

Ultimately we should try to open the device exclusively so that no other process can have the device open to cause this error, but this is a more difficult problem.

[jellyfish-bot]

[pipex] This issue has attached support thread https://jel.ly.fish/4ce48cd0-5b76-45f5-a86f-00b4fca21b1d