Use dedicated exceptions if unlinking failed for reasons other than the

file not existing

src/MD_STD.php

@@ -110,7 +110,15 @@ final class MD_STD {
     public static function unlink(string $filename):void {
 
         if (\unlink($filename) === false) {
-            throw new MDFileDoesNotExist("Failed to delete: $filename");
+            if (!\is_file($filename)) {
+                throw new MDFileDoesNotExist("Failed to delete $filename, it did not exist");
+            }
+            else if (!\is_writable(dirname($filename))) {
+                throw new MDFilePermissionsException("Failed to delete $filename");
+            }
+            else {
+                throw new MDFailedToDeleteFile("Failed to delete $filename");
+            }
         }
 
     }