faft: Show the RPC address in some socket errors.

Before, it could be unclear what connection was refused:
[Errno 111] Connection refused

Now, the port number suggests that it's a local servod:
[Errno 111] Connection refused: 'localhost:9999'

BUG=None
TEST=Run firmware_FAFTSetup with local servod not running

Change-Id: I840fc792142662ce0cd82def3d0b506ddc936dc7
Reviewed-on: https://chromium-review.googlesource.com/1682167
Tested-by: Dana Goyette <[email protected]>
Commit-Ready: Dana Goyette <[email protected]>
Legacy-Commit-Queue: Commit Bot <[email protected]>
Reviewed-by: Greg Edelston <[email protected]>
diff --git a/server/hosts/rpc_server_tracker.py b/server/hosts/rpc_server_tracker.py
index 5dbb719..feb7890 100644
--- a/server/hosts/rpc_server_tracker.py
+++ b/server/hosts/rpc_server_tracker.py
@@ -189,13 +189,20 @@
                          delay_sec=min(max(timeout_seconds / 20.0, 0.1), 1))
             def ready_test():
                 """ Call proxy.ready_test_name(). """
-                getattr(proxy, ready_test_name)()
+                try:
+                    getattr(proxy, ready_test_name)()
+                except socket.error as e:
+                    e.filename = rpc_url.replace('http://', '')
+                    raise
             successful = False
             try:
                 logging.info('Waiting %d seconds for XMLRPC server '
                              'to start.', timeout_seconds)
                 ready_test()
                 successful = True
+            except socket.error as e:
+                e.filename = rpc_url.replace('http://', '')
+                raise
             finally:
                 if not successful:
                     logging.error('Failed to start XMLRPC server.')