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>

test/parallel/test-console-instance.js

@@ -1,10 +1,13 @@
 'use strict';
 require('../common');
-var assert = require('assert');
-var Stream = require('stream');
-var Console = require('console').Console;
+const assert = require('assert');
+const Stream = require('stream');
+const Console = require('console').Console;
 var called = false;
 
+const out = new Stream();
+const err = new Stream();
+
 // ensure the Console instance doesn't write to the
 // process' "stdout" or "stderr" streams
 process.stdout.write = process.stderr.write = function() {
@@ -20,9 +23,13 @@ assert.throws(function() {
   new Console();
 }, /Console expects a writable stream/);
 
-var out = new Stream();
-var err = new Stream();
-out.writable = err.writable = true;
+// Console constructor should throw if stderr exists but is not writable
+assert.throws(function() {
+  out.write = function() {};
+  err.write = undefined;
+  new Console(out, err);
+}, /Console expects writable stream instances/);
+
 out.write = err.write = function(d) {};
 
 var c = new Console(out, err);