A number is not runnable and so should act as if the procedure is unknown.
Previously, this threw an error during treeification, but a recent bug fix made it so that treeification never throws an error.
That fix introduced a crash when executing a statement like "(2 1)" because the
downstream logic didn't handle when a number was given as a procedure.
This change restores the previous behavior of throwing a "I don't know how to" error, except that it's now recoverable.
