child_process: improve maxBuffer error message

Mention what buffer (stdout, stderr) overflowed.

lib/child_process.js

@@ -574,7 +574,7 @@ exports.execFile = function(file /* args, options, callback */) {
   child.stdout.addListener('data', function(chunk) {
     stdout += chunk;
     if (stdout.length > options.maxBuffer) {
-      err = new Error('maxBuffer exceeded.');
+      err = new Error('stdout maxBuffer exceeded.');
       kill();
     }
   });
@@ -582,7 +582,7 @@ exports.execFile = function(file /* args, options, callback */) {
   child.stderr.addListener('data', function(chunk) {
     stderr += chunk;
     if (stderr.length > options.maxBuffer) {
-      err = new Error('maxBuffer exceeded.');
+      err = new Error('stderr maxBuffer exceeded.');
       kill();
     }
   });