Better warning messages when uploading mulitple trees in one newick file

[mooreryan]

If you upload a newick file that has mulitple trees...eg a consensus tree followed by the bootstrap trees, you won't be able to view the bootstrap support values as Iroki only plots the last tree in a file.

Add this info to the warning that is shown.

Long term, it would probably be better for the parser to return the first tree in the file rather than the last one.