Don't stop volume service for failed re-export operations

Commit b71570 "Set vol driver initialized before deleting volumes"
changed the manager behavior to call set_initialized() before
attempting to re-export volumes.

Drivers should not be considered initialized before re-export has
succeeded.  Otherwise a failure to export causes the volume
service to stop, when that failure should be handled like any other
failure to initialize.

Closes-Bug: 1257049
Related-Bug: 1232177
Change-Id: Ic6bc89ef3f15dbbc971fdd8c91117cccb5c2801b
Merge "Don't stop volume service for failed re-export operations"