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/nan/tools/README.md

@@ -0,0 +1,14 @@
+1to2 naively converts source code files from NAN 1 to NAN 2. There will be erroneous conversions,
+false positives and missed opportunities. The input files are rewritten in place. Make sure that
+you have backups. You will have to manually review the changes afterwards and do some touchups.
+
+```sh
+$ tools/1to2.js
+
+  Usage: 1to2 [options] <file ...>
+
+  Options:
+
+    -h, --help     output usage information
+    -V, --version  output the version number
+```