tls-psk: show error message for unspecialized io exception

OpenKeychain/src/main/res/values/strings.xml

@@ -1901,6 +1901,8 @@
     <string name="transfer_error_read_incoming">"Failed reading incoming key!"</string>
     <string name="transfer_error_connect">"Connection failed!"</string>
     <string name="transfer_error_listen">"Error waiting for connection!"</string>
+    <string name="transfer_error_generic">"Unknown communication error!"</string>
+    <string name="transfer_error_generic_msg">"Communication error: %s"</string>
     <string name="transfer_not_available">"Sorry, this feature can only be used on Android 5 or newer :("</string>
     <string name="transfer_list_empty">No keys on this device that could be sent.  Waiting for incoming keys…</string>