exec node returns 0 on the third output if command ended without error. (#1160)

* exec node returns 0 on the third output if command ended without error.
Otherwise, the status of the node is updated and the error code is send through the third output.

* info text updated and the second output returns only something if stderr is not empty

* proper stderror handling

* proper handling of stderr

nodes/core/core/75-exec.html

@@ -47,10 +47,13 @@
     <p>Calls out to a system command.<br/></p>
     <p>Provides 3 outputs: stdout, stderr, and return code.</p>
     <p>By default uses the <code>exec</code> system call which calls the command, then gets a callback
-    on completion, returning the complete result in one message, along with any errors.</p>
+    on completion, returning stdout as the payload to the first, the error code as the third
+    and, if available, stderr to the second output. If no error occurred, a zero is returned on the third output.</p>
     <p>Optionally can use <code>spawn</code> instead, which returns the output from stdout and stderr
     as the command runs (usually one line at a time). On completion it then returns a return code
     (on the 3rd output).</p>
+    <p>The <code>exec</code> method spawns a subshell and therefore can be used for more complicated
+    commands involving pipes. However, it waits for completion of the whole command before returing anything.</p> 
     <p>The optional append gets added to the command after <code>msg.payload</code> - so you can do
     things like pipe the result to another command.</p>
     <p>Commands or parameters with spaces should be enclosed in quotes - <i>"This is a single parameter"</i></p>

nodes/core/core/75-exec.js

@@ -102,15 +102,19 @@ module.exports = function(RED) {
                 child = exec(cl, {encoding: 'binary', maxBuffer:10000000}, function (error, stdout, stderr) {
                     msg.payload = new Buffer(stdout,"binary");
                     if (isUtf8(msg.payload)) { msg.payload = msg.payload.toString(); }
-                    var msg2 = {payload:stderr};
-                    var msg3 = null;
+                    var msg2 = null;
+                    if(stderr) {
+                        msg2 = {payload: stderr};
+                    }
+                    var msg3 = {payload:0};
+                    node.status({});
                     //console.log('[exec] stdout: ' + stdout);
                     //console.log('[exec] stderr: ' + stderr);
                     if (error !== null) {
-                        msg3 = {payload:error};
+                        msg3 = {payload:error.code};
+                        node.status({fill:"red",shape:"dot",text:"error: "+error.code});
                         //console.log('[exec] error: ' + error);
                     }
-                    node.status({});
                     node.send([msg,msg2,msg3]);
                     if (child.tout) { clearTimeout(child.tout); }
                     delete node.activeProcesses[child.pid];