PRS505/700: Dont fail to detect the device if SD card is buggy

src/calibre/devices/prs505/driver.py

@@ -248,6 +248,7 @@ class PRS505(Device):
                 time.sleep(3)
                 self.open_osx()
         if self._card_prefix is not None:
+            try:
                 cachep = os.path.join(self._card_prefix, self.CACHE_XML)
                 if not os.path.exists(cachep):
                     os.makedirs(os.path.dirname(cachep), mode=0777)
@@ -257,6 +258,10 @@ class PRS505(Device):
 </cache>
 '''.encode('utf8'))
                     f.close()
+            except:
+                self._card_prefix = None
+                import traceback
+                traceback.print_exc()
             
     def set_progress_reporter(self, pr):
         self.report_progress = pr