Trap insufficient-taxa error and provide useful message

[arlin]

This is the follow-up to phylotastic-portal#330, tree retrieval issue.

If the user's list does not match enough nodes in the synthetic tree, then no tree can be returned.

Providing the message from the underlying service is useful, but only for a minority of users. Most users will not understand why their list is not working. We need to tell them that this error often happens because the species names in a resource do not match OT's taxonomy.

We need to get more information on why cases fail so that we can craft the right message.