python/aqmp: fix ConnectError string method

When ConnectError is used to wrap an Exception that was initialized
without an error message, we are treated to a traceback with a rubbish
line like this:

... ConnectError: Failed to establish session:

Correct this to use the name of an exception as a fallback message:

... ConnectError: Failed to establish session: EOFError

Better!

Signed-off-by: John Snow <jsnow@redhat.com>
Reported-by: Thomas Huth <thuth@redhat.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Message-id: 20211111143719.2162525-3-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>

python/qemu/aqmp/protocol.py

@@ -79,7 +79,11 @@ def __init__(self, error_message: str, exc: Exception):
         self.exc: Exception = exc
 
     def __str__(self) -> str:
-        return f"{self.error_message}: {self.exc!s}"
+        cause = str(self.exc)
+        if not cause:
+            # If there's no error string, use the exception name.
+            cause = exception_summary(self.exc)
+        return f"{self.error_message}: {cause}"
 
 
 class StateError(AQMPError):