Trees with polytomies give opaque warning "ERROR: tree std::string"

I inadvertently gave bamm a tree with a polytomous branching. The error message was "ERROR: tree std::string" - which I think comes from Line 740 of Tree.cpp (and not, say, L748 or L756). After much puzzling I figured out that the tree was not binary. Passing the tree through R/ape's _multi2di_ function allows bamm to  work. 

The attached pull request just makes this error message a bit more useful.