Increase flexibility of CA sequences merging

In HLT use-case we have sometimes situation when the signatures code need to provide algorithms that need to be inserted in HLT CF and some other in the top-level sequences structure. This MR enables it. Simple the code needs to generate a CA with multiple sequences. The main one is used in CF while all others are eventually added to the tree somewhere else.

There is an example (that will be uncommented in the follow-up MR) of how it will be used in a particular case of providing BCIDAvg to all HLT calo algorithms.

Also, improved a bit of messaging and suppressed unmerged CA deletion errors when there is an exception. Tagging @wlampl