Closes GH-85 Emit error rather than throwing.

Since "error" events will throw when unhandled anyhow, it makes no sense
to throw from an EventEmitter's method, especially for such a minor
misdemeanor as attempting to write to a non-writable stream.

lib/fs.js

@@ -1027,7 +1027,8 @@ WriteStream.prototype.flush = function() {
 
 WriteStream.prototype.write = function(data) {
   if (!this.writable) {
-    throw new Error('stream not writable');
+    this.emit("error", new Error('stream not writable'));
+    return false;
   }
 
   this.drainable = true;

lib/tty_win32.js

@@ -99,7 +99,8 @@ WriteStream.prototype.isTTY = true;
 
 WriteStream.prototype.write = function(data, encoding) {
   if (!this.writable) {
-    throw new Error('stream not writable');
+    this.emit("error", new Error('stream not writable'));
+    return false;
   }
 
   if (Buffer.isBuffer(data)) {