[MERGE][220067] add more specificity to the error message when split fails

[Neil Martinsen-Burrell]

Bug 220067 is about a misleading upgrade message when split fails 
because the branch does not support rich roots.  The attached changes 
the message to specifically mention upgrading to support rich roots.  It 
does not mention any formats by name since those may change (hopefully 
soon once and for all).  The help for the split command does list the 
needed formats, so I think it is reasonable to expect people to look there.

-Neil
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