Failure to find an oem partition should not be a remount failure.

Many devices don't have an /oem partition, so find_mount should be
expected to fail, but shouldn't cause the overall remount to fail.

Also clean up all the error handling and reporting, and remove the
dead int* globals.

Bug: http://b/21024141
Change-Id: Ie31021b03c9cab8e972269d7d1ffe383cd30ee9e