Using the in the the wrong parameter with the SPWeb.GetList method you will receive a FileNotFoundException with the following very SharePoint-ish error message:
The stacktrace will often give more information but the ULS log it wil simply show:
So the parameter is incorrect; Either the URL does not specify a valid path to the website, or the list does not exist in the website.
The GetList method gets the list that is associated with the specified server-relative URL to the root folder of a list, such as /sites/sitecollection/subsite/Lists/Announcements.
You can use the SPUrlUtility.CombineUrl with your webs server relative URL like this:
var list = web.GetList(SPUrlUtility.CombineUrl(web.ServerRelativeUrl,
"lists/" + listFolderName));
Be aware; there are other methods casting System.IO.FileNotFoundException!