debugger: improve ESRCH error message

When use `iojs debug -p <pid>` with an invalid pid,
the debugger print internal error message also,
it not enough smart.

lib/_debugger.js

@@ -1639,7 +1639,16 @@ Interface.prototype.trySpawn = function(cb) {
   } else if (this.args.length === 3) {
     // `node debug -p pid`
     if (this.args[1] === '-p' && /^\d+$/.test(this.args[2])) {
-      process._debugProcess(parseInt(this.args[2], 10));
+      const pid = parseInt(this.args[2], 10);
+      try {
+        process._debugProcess(pid);
+      } catch (e) {
+        if (e.code === 'ESRCH') {
+          console.error('Target process: ' + pid + ' doesn\'t exist.');
+          process.exit(1);
+        }
+        throw e;
+      }
       isRemote = true;
     } else {
       var match = this.args[1].match(/^--port=(\d+)$/);
@@ -1705,7 +1714,7 @@ Interface.prototype.trySpawn = function(cb) {
     // If it's failed to connect 10 times then print failed message
     if (connectionAttempts >= 10) {
       self.stdout.write(' failed, please retry\n');
-      return;
+      process.exit(1);
     }
     setTimeout(attemptConnect, 500);
   }