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/interfaces

@@ -0,0 +1,23 @@
+{
+    "extends":"http://json-schema.org/hyper-schema",
+    "description":"A schema for schema interface definitions that describe programmatic class structures using JSON schema syntax",
+    "properties":{
+        "methods":{
+            "type":"object",
+            "description":"This defines the set of methods available to the class instances",
+            "additionalProperties":{
+            	"type":"object",
+            	"description":"The definition of the method",
+            	"properties":{
+            		"parameters":{
+            			"type":"array",
+            			"description":"The set of parameters that should be passed to the method when it is called",
+            			"items":{"$ref":"#"},
+            			"required": true
+            		},
+            		"returns":{"$ref":"#"}
+            	}
+            }
+        }    
+    }
+}