Make the exception a bit more explicit about what is wrong

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1424281 13f79535-47bb-0310-9956-ffa450edef68

src/java/org/apache/poi/poifs/storage/HeaderBlock.java

@@ -136,10 +136,11 @@ public final class HeaderBlock implements HeaderBlockConstants {
 						+ "POI only supports BIFF8 format");
 			}
 
-			// Give a generic error
+			// Give a generic error if the OLE2 signature isn't found
 			throw new IOException("Invalid header signature; read "
 				                  + longToHex(signature) + ", expected "
-				                  + longToHex(_signature));
+				                  + longToHex(_signature) + " - Your file appears "
+				                  + "not to be a valid OLE2 document");
 		}