encoding: make getcols() raise exception explicitly

This helps pytype.

  File "mercurial/encoding.py", line 329, in getcols: bad option in return
  type [bad-return-type]
  Expected: bytes
  Actually returned: None

mercurial/encoding.py

@@ -327,6 +327,7 b' def getcols(s, start, c):'
         t = s[start:x]
         if colwidth(t) == c:
             return t
+    raise ValueError('substring not found')
 
 
 def trim(s, width, ellipsis=b'', leftside=False):