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/json-schema/draft-03/examples/address

@@ -0,0 +1,20 @@
+{
+	"description" : "An Address following the convention of http://microformats.org/wiki/hcard",
+	"type" : "object",
+	"properties" : {
+		"post-office-box" : { "type" : "string" },
+		"extended-address" : { "type" : "string" },
+		"street-address" : { "type":"string" },
+		"locality" : { "type" : "string", "required" : true },
+		"region" : { "type" : "string", "required" : true },
+		"postal-code" : { "type" : "string" },
+		"country-name" : { "type" : "string", "required" : true }
+	},
+	"dependencies" : {
+		"post-office-box" : "street-address",
+		"extended-address" : "street-address",
+		"street-address" : "region",
+		"locality" : "region",
+		"region" : "country-name"
+	}
+}