Check if taxon is present in tree for add_cherry_to_tip() and produce informative error message.

When trying to add new tips next to a taxon that is not present in tree, add_cherry_to_tip() crashes with the following error:

`Error in if (where == 0 || where == "root") where <- ROOTx : 
  missing value where TRUE/FALSE needed`

A test if the taxon the function is looking for with a helpful error message if it is not present would be nice.