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/har-validator/lib/error.js

@@ -0,0 +1,17 @@
+function HARError (errors) {
+  var message = 'validation failed'
+
+  this.name = 'HARError'
+  this.message = message
+  this.errors = errors
+
+  if (typeof Error.captureStackTrace === 'function') {
+    Error.captureStackTrace(this, this.constructor)
+  } else {
+    this.stack = (new Error(message)).stack
+  }
+}
+
+HARError.prototype = Error.prototype
+
+module.exports = HARError