Added checks to make sure the specified track exists

Added precondition checks to validate input files and paths to return more informative error messages as per #2

node_modules/@cnakazawa/watch/test/test_monitor.js

@@ -0,0 +1,31 @@
+var watch = require('../main')
+  , assert = require('assert')
+  , path = require('path')
+  , fs = require('fs')
+  , target = path.join(__dirname, "d/t")
+  ;
+
+function clearFile() {
+  fs.writeFileSync(target, '')
+}
+
+clearFile()
+
+// test if changed event is fired correctly
+watch.createMonitor(__dirname, { interval: 150 },
+  function (monitor) {
+    monitor.once('changed', function (f) {
+      assert.equal(f, target);
+      clearFile();
+      process.exit(0)
+    })
+
+    fs.writeFile(target, 'Test Write\n', function (err) {
+      if (err) throw err;
+
+      setTimeout(function () {
+        // should have got the other assert done by now
+        assert.ok(false);
+      }, 300);
+    })
+});