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/whatwg-encoding/lib/supported-names.json

@@ -0,0 +1,37 @@
+[
+  "UTF-8",
+  "IBM866",
+  "ISO-8859-2",
+  "ISO-8859-3",
+  "ISO-8859-4",
+  "ISO-8859-5",
+  "ISO-8859-6",
+  "ISO-8859-7",
+  "ISO-8859-8",
+  "ISO-8859-10",
+  "ISO-8859-13",
+  "ISO-8859-14",
+  "ISO-8859-15",
+  "ISO-8859-16",
+  "KOI8-R",
+  "KOI8-U",
+  "macintosh",
+  "windows-874",
+  "windows-1250",
+  "windows-1251",
+  "windows-1252",
+  "windows-1253",
+  "windows-1254",
+  "windows-1255",
+  "windows-1256",
+  "windows-1257",
+  "windows-1258",
+  "GBK",
+  "gb18030",
+  "Big5",
+  "EUC-JP",
+  "Shift_JIS",
+  "EUC-KR",
+  "UTF-16BE",
+  "UTF-16LE"
+]