filesystem: fix exception string (fid != name)

It is better to use the term "fid" instead of "name" when a reserved FID
is detected.

Change-Id: I054f3b3a156f0164c62610cfde1aec2145c20925
Related: OS#4963

pySim/filesystem.py

@@ -143,7 +143,7 @@ class CardDF(CardFile):
         if child.name in CardFile.RESERVED_NAMES:
             raise ValueError("File name %s is a reserved name" % (child.name))
         if child.fid in CardFile.RESERVED_FIDS:
-            raise ValueError("File fid %s is a reserved name" % (child.fid))
+            raise ValueError("File fid %s is a reserved fid" % (child.fid))
         if child.fid in self.children:
             if ignore_existing:
                 return