Reorder isMounted for readability

[wongma7]

Is this a bug fix or adding new feature?

What is this PR about? / Why do we need it? my addition #1081 made this function a bit confusing #1083 (comment) so want to do some quick refactoring before I forget what intention of my change was.

• make return value explicit for each case
  • if returning error, just return false. The caller should check the error and ignore the boolean.
  • if target DNE, just return false. IsLikelyNotMountPoint must have returned "true, ErrNotExists", so returning "false, nil" is the same as returning "!notMnt, nil" like this function was doing before
• add debug log message for case when target path doesn't exist (on Windows it would be annoying at lower verbosity cuz it would get printed all the time
• make isMounted come right after preparePublishPath in both the block and non-block case.

What testing is done?

[wongma7]

/cc @nckturner

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[wongma7]

it might be clearer to just make isMounted platform specific.

[nirmalaagash]

/lgtm