The error
  "Unsupported collation on string indexed column %s Use
    binary collation (latin1_bin, binary, utf8_bin)."

is misleading. Change it:
- It is now a warning
- It is printed only for collations that do not support index-only access
  (reversible collations that use unpack_info are ok)
- The new warning text is:

  Indexed column %s.%s uses a collation that does not allow index-only
  access in secondary key and has reduced disk space efficiency
  in primary key.