magic-proxy: fix exception handling for URLError LP:#1946520

A urllib.error.URLError.reason variable can either be a string or
another Exception[0]. In case it's another exception, the current code
fails because the exception is passed into send_error() which tries
call html.escape() on the Exception. That fails because the Exception
is not a string. Converting the Exception to a string fixes this.

This fixes:

AttributeError: 'TimeoutError' object has no attribute 'replace'

[0]
https://docs.python.org/3/library/urllib.error.html#urllib.error.URLError.reason

(cherry picked from commit af888e24ff)

magic-proxy

@@ -883,8 +883,10 @@ class ProxyingHTTPRequestHandler(http.server.BaseHTTPRequestHandler):
             self.__send_response(e)
         except urllib.error.URLError as e:
             self.log_message(
-                "urlopen() failed for {} with {}".format(uri, e.reason))
-            self.send_error(501, e.reason)
+                "urlopen() failed for {} with {}".format(uri, str(e.reason)))
+            # URLError.reason can either be a string or another Exception
+            # So do convert it to a string before sending the error (LP: #1946520)
+            self.send_error(501, str(e.reason))
 
 
     def __get_host_path(self):