ENH: Raise error in certain unhandled _reduce cases.
Oh, I’d thought there was possibly interest in supporting axis 1 (single element reduction along a perpendicular axis) because the preexisting group by tests I modified had been attempting to sum a series on axis 1.

I changed to make this a value error per your comment.
 pls rebase and add a doc-note in v0.15.1 enhancements (reffing this pr as it doesn't have a separate issue)
 Ok, rebased and added to whatsnew.
 @staple sorry, you will have to rebase again, I moved the whatsnew file to a subfolder
 @staple pls squash as well, otherwise looks ok
 Ok, rebased and squashed.
 minor comment: is this error message only supposed to be raised internally? Or will users also see this in some cases? 
If that is the case (users will see it), I don't think it is clear as users don't know what `_reduce` is. If it is not the case, the enhancement entry seems not very interesting for users (not to say that the PR would not be interesting of course!)
 @jorisvandenbossche Users can see this error message. What would make more sense for a user message? Would something like 'Series aggregation does not implement numeric_only' make sense?
 Would it be possible to detect in which kind of aggregation function this is called? 
 It looks like 'name' (the name of the aggregation function) is generally passed to _reduce. But it's an optional argument. I could try to make it a non optional argument, and use the value there in the error message. Would that make sense?
 I just wanted to suggest the same!
 Ok, cool.
 It is indeed an optional argument, bug eg in https://github.com/staple/pandas/blob/reduce/pandas/core/generic.py#L3924 where it is used in `_make_stat_function`, `name` is not an optional argument, so it would always be present (but you should look if it is used in other places)
 Ok, I made the requested changes. Left them in separate commits for now, for ease of review. But if you like I can squash them when you're ready. Let me know if the change to make 'name' a required argument to _reduce should be a separate commit.
 Ok changed the whats new text.
 Yep, better! 
@jreback OK with you that `name` is now a required positional arg of `_reduce`?
 let me have a look
 Another remark: shouldn't this rather be a UserWarning instead of an error? (I don't know how similar things are handled in other places in pandas) 
Also, if we raise an error and not a warning, we should maybe keep this for 0.16 instead of 0.15.1?
 I think this is ok (only thing, maybe need a couple of specific cases that actually trigger this error, rather than a constructed case). can't think of any off-hand though. pls rebase / squash.
 Ok, I rebased and squashed.
 I think this is fine. @jorisvandenbossche ?
 thanks!
