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/ajv/lib/dot/pattern.jst

@@ -0,0 +1,14 @@
+{{# def.definitions }}
+{{# def.errors }}
+{{# def.setupKeyword }}
+{{# def.$data }}
+
+{{
+  var $regexp = $isData
+                ? '(new RegExp(' + $schemaValue + '))'
+                : it.usePattern($schema);
+}}
+
+if ({{# def.$dataNotType:'string' }} !{{=$regexp}}.test({{=$data}}) ) {
+  {{# def.error:'pattern' }}
+} {{? $breakOnError }} else { {{?}}