Provide a better diagnosis on socket errors

Lib/smtplib.py

@@ -364,8 +364,10 @@ class SMTP:
         while 1:
             try:
                 line = self.file.readline()
-            except socket.error:
-                line = ''
+            except socket.error as e:
+                self.close()
+                raise SMTPServerDisconnected("Connection unexpectedly closed: "
+                                             + str(e))
             if not line:
                 self.close()
                 raise SMTPServerDisconnected("Connection unexpectedly closed")