transport/pcsc: throw ReaderError with a message

Before this patch:

  $ ./pySim-shell.py -p 0
  Card reader initialization failed with an exception of type:
  <class 'pySim.exceptions.ReaderError'>

after:

  $ ./pySim-shell.py -p 0
  Card reader initialization failed with exception:
  No reader found for number 0

Change-Id: Id08c4990857f7083a8d1cefc90ff85fc20ab6fef

pySim/transport/pcsc.py

@@ -34,7 +34,7 @@ class PcscSimLink(LinkBase):
         super().__init__(**kwargs)
         r = readers()
         if reader_number >= len(r):
-            raise ReaderError
+            raise ReaderError('No reader found for number %d' % reader_number)
         self._reader = r[reader_number]
         self._con = self._reader.createConnection()