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/pump/test-node.js

@@ -0,0 +1,53 @@
+var pump = require('./index')
+
+var rs = require('fs').createReadStream('/dev/random')
+var ws = require('fs').createWriteStream('/dev/null')
+
+var toHex = function () {
+  var reverse = new (require('stream').Transform)()
+
+  reverse._transform = function (chunk, enc, callback) {
+    reverse.push(chunk.toString('hex'))
+    callback()
+  }
+
+  return reverse
+}
+
+var wsClosed = false
+var rsClosed = false
+var callbackCalled = false
+
+var check = function () {
+  if (wsClosed && rsClosed && callbackCalled) {
+    console.log('test-node.js passes')
+    clearTimeout(timeout)
+  }
+}
+
+ws.on('close', function () {
+  wsClosed = true
+  check()
+})
+
+rs.on('close', function () {
+  rsClosed = true
+  check()
+})
+
+var res = pump(rs, toHex(), toHex(), toHex(), ws, function () {
+  callbackCalled = true
+  check()
+})
+
+if (res !== ws) {
+  throw new Error('should return last stream')
+}
+
+setTimeout(function () {
+  rs.destroy()
+}, 1000)
+
+var timeout = setTimeout(function () {
+  throw new Error('timeout')
+}, 5000)