Cbbackupmgr: Investigate why Alice and Vulcan incorrectly report successfully merge on failure

Problem
There has been a few cases where cbbackupmgr incorrect reports a successfully merge when there has been a failure (see MB-36248). There is a now reproduction of this issue and we should investigate if cbbackupmgr is not checking the error correct or if forestdb is not passing the error up. I suspect that this does not affect MH as the code has been changed.

Expectation
For the error to passed up and the merge to fail.

Update
After investigating the error further is seems that FDB is not passing the error back so cbbackupmgr is unaware of the error, please see the comments below for more details.