stream: improve stream error messages

Improve message when tranform._transform() method is not implemented
Improve error message when Readable._read() is not implemented
Remove extra word in err msg when Writable._write() when not implemented
Remove extra word in err msg when Transform._transform() when not implemented

PR-URL: #8801
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>

lib/_stream_readable.js

@@ -467,7 +467,7 @@ function maybeReadMore_(stream, state) {
 // for virtual (non-string, non-buffer) streams, "length" is somewhat
 // arbitrary, and perhaps not very meaningful.
 Readable.prototype._read = function(n) {
-  this.emit('error', new Error('not implemented'));
+  this.emit('error', new Error('_read() is not implemented'));
 };
 
 Readable.prototype.pipe = function(dest, pipeOpts) {

lib/_stream_transform.js

@@ -139,7 +139,7 @@ Transform.prototype.push = function(chunk, encoding) {
 // an error, then that'll put the hurt on the whole operation.  If you
 // never call cb(), then you'll never get another chunk.
 Transform.prototype._transform = function(chunk, encoding, cb) {
-  throw new Error('Not implemented');
+  throw new Error('_transform() is not implemented');
 };
 
 Transform.prototype._write = function(chunk, encoding, cb) {

lib/_stream_writable.js

@@ -432,7 +432,7 @@ function clearBuffer(stream, state) {
 }
 
 Writable.prototype._write = function(chunk, encoding, cb) {
-  cb(new Error('_write() method is not implemented'));
+  cb(new Error('_write() is not implemented'));
 };
 
 Writable.prototype._writev = null;