fix to throw NullPointerException instead of IOException

src/prettify/SyntaxHighlighter.java

diff --git a/src/prettify/SyntaxHighlighter.java b/src/prettify/SyntaxHighlighter.java
index 8c109d4..c0dd605 100644
--- a/src/prettify/SyntaxHighlighter.java
+++ b/src/prettify/SyntaxHighlighter.java
@@ -285,7 +285,7 @@
    */
   protected static String readFile(File file) throws IOException {
     if (file == null) {
-      throw new IOException("argument 'file' cannot be null");
+      throw new NullPointerException("argument 'file' cannot be null");
     }
 
     byte[] buffer = new byte[(int) file.length()];