A ProtocolError object describes a protocol error in the
underlying transport layer (such as a 404 `not found' error if the
server named by the URI does not exist). It has the following
members:
url
The URI or URL that triggered the error.
errcode
The error code.
errmsg
The error message or diagnostic string.
headers
A string containing the headers of the HTTP/HTTPS request that
triggered the error.
