BUG: merge_ordered should raise when elements in by not exist in df

[GYHHAHA]

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• (optional) I have confirmed this bug exists on the master branch of pandas.

---

>>>l = pd.DataFrame([['g', 'h', 1], ['g', 'h', 3]], columns=list('GHT'))
>>>l
   G  H  T
0  g  h  1
1  g  h  3
>>>r = pd.DataFrame([[2, 1]], columns=list('TE'))
>>>r
   T  E
0  2  1

Result

>>>pd.merge_ordered(l, r, on='T', left_by=['G', 'h'])

	G	H	T	E	h
0	G	h	1	nan	G
1	G	nan	2	1	G
2	h	nan	2	1	h
3	h	h	3	nan	h

expected

>>>pd.merge_ordered(l, r, on='T', left_by=['G', 'h'])
KeyError: "['h'] not found in left columns"