don't check error code of `diskutil eject`

nstead, determine success by directly testing the intended effect:
does the mount path still exist?

Also:
 - retry once on failure.
 - silent success if the given mount point did not exist.

These changes are intended to help with unpredictable problems with
the test suite that manifest frequently on Travis.

References: #4975, #4900, #4857
Merge pull request #4978 from rolandwalker/eject_error_code

don't check error code of diskutil eject