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/fsevents/binding.gyp

@@ -0,0 +1,29 @@
+{
+  "targets": [
+    { "target_name": "" }
+  ],
+  "conditions": [
+    ['OS=="mac"', {
+      "targets": [{
+        "target_name": "<(module_name)",
+        "sources": ["fsevents.cc"],
+        "xcode_settings": {
+          "OTHER_LDFLAGS": [
+            "-framework CoreFoundation -framework CoreServices"
+          ]
+        },
+        "include_dirs": [
+          "<!(node -e \"require('nan')\")"
+        ]
+      }, {
+        "target_name": "action_after_build",
+        "type": "none",
+        "dependencies": ["<(module_name)"],
+        "copies": [{
+          "files": ["<(PRODUCT_DIR)/<(module_name).node"],
+          "destination": "<(module_path)"
+        }]
+      }]
+    }]
+  ]
+}