console: check that stderr is writable

console: check that stderr is writable

`Console` constructor checks that `stdout.write()` is a function but
does not do an equivalent check for `stderr.write()`. If `stderr` is not
specified in the constructor, then `stderr` is set to be `stdout`.
However, if `stderr` is specified, but `stderr.write()` is not a
function, then an exception is not thrown until `console.error()` is
called.

This change adds the same check for 'stderr' in the constructor that is
there for `stdout`. If `stderr` fails the check, then a `TypeError` is
thrown.

Took the opportunity to copyedit the `console` doc a little too.

PR-URL: https://github.com/nodejs/node/pull/5635
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>