remove dead require

Dumb mistake.
move method to FetchIn module
require at least one key argument
don't explicitly rescue KeyError

It's a subclass of IndexError.  In fact, `Hash#fetch` only started
raising KeyError and not IndexError in 1.9.
'raises a $err' -> 'raises $err'

cf grammar, the english language, etc.
prefer `fail` to `raise`
kernel-fetch_in 0.2.0

* require at least one key argument.
* move method to FetchIn module.
* remove some dead code
* use better english