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/_limitLength.jst

@@ -0,0 +1,10 @@
+{{# def.definitions }}
+{{# def.errors }}
+{{# def.setupKeyword }}
+{{# def.$data }}
+
+{{ var $op = $keyword == 'maxLength' ? '>' : '<'; }}
+if ({{# def.$dataNotType:'number' }} {{# def.strLength }} {{=$op}} {{=$schemaValue}}) {
+  {{ var $errorKeyword = $keyword; }}
+  {{# def.error:'_limitLength' }}
+} {{? $breakOnError }} else { {{?}}