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_watchTree.js

@@ -0,0 +1,23 @@
+var fs = require('fs')
+  , watch = require('../main')
+  , assert = require('assert')
+  ;
+
+//
+// Demonstrate that the function of 'filter' is semantically inconsistent with
+// usual convention, that returning true means 'keep this'.
+//
+function isDirOrQ(f, stat) { return stat.isDirectory() || f === 'Q'; }
+
+watch.watchTree(__dirname, { filter: isDirOrQ }, function (f, curr, prev) {
+  if (typeof f == 'object' && prev === null && curr === null) {
+    Object.keys(f).forEach(function(name) {
+      var stat = f[name];
+      assert(isDirOrQ(name, stat));
+    });
+
+    // If the process never exits then `unwatchTree` failed to unwatch all
+    // the files.
+    watch.unwatchTree(__dirname);
+  }
+});