Pass in more structured context into exception

This may help us provide better error messages if needed, instead of
being stuck with an eagerly-rendered string.

src/pip/_internal/exceptions.py

@@ -12,7 +12,6 @@
     from typing import Any, Optional, List, Dict, Text
 
     from pip._vendor.pkg_resources import Distribution
-    from pip._vendor.requests.models import Response, Request
     from pip._vendor.six import PY3
     from pip._vendor.six.moves import configparser
 
@@ -103,24 +102,32 @@ class PreviousBuildDirError(PipError):
 class NetworkResponseError(PipError):
     """HTTP connection error"""
 
-    def __init__(self, error_msg, response=None, request=None):
-        # type: (Text, Response, Request) -> None
-        """
-        Initialize NetworkResponseError with  `request` and `response`
-        objects.
-        """
-        self.response = response
-        self.request = request
-        self.error_msg = error_msg
-        if (self.response is not None and not self.request and
-                hasattr(response, 'request')):
-            self.request = self.response.request
-        super(NetworkResponseError, self).__init__(
-            error_msg, response, request)
+    error_type = "Unknown Error"
+
+    def __init__(self, status, reason, url):
+        # type: (int, Text, str) -> None
+        super(NetworkResponseError, self).__init__(status, reason, url)
+        self.status = status
+        self.reason = reason
+        self.url = url
 
     def __str__(self):
         # type: () -> str
-        return ensure_str(self.error_msg, errors="replace")
+        message = u"{} {}: {} for url: {}".format(
+            self.status,
+            self.error_type,
+            self.reason,
+            self.url,
+        )
+        return ensure_str(message, errors="replace")
+
+
+class NetworkResponseClientError(NetworkResponseError):
+    error_type = "Client Error"
+
+
+class NetworkResponseServerError(NetworkResponseError):
+    error_type = "Server Error"
 
 
 class InvalidWheelFilename(InstallationError):

src/pip/_internal/network/download.py

@@ -169,10 +169,7 @@ def __call__(self, link):
         try:
             resp = _http_get_download(self._session, link)
         except NetworkResponseError as e:
-            assert e.response is not None
-            logger.critical(
-                "HTTP error %s while getting %s", e.response.status_code, link
-            )
+            logger.critical("HTTP error %s while getting %s", e.status, link)
             raise
 
         return Download(

src/pip/_internal/network/utils.py

@@ -1,6 +1,9 @@
 from pip._vendor.requests.models import CONTENT_CHUNK_SIZE, Response
 
-from pip._internal.exceptions import NetworkResponseError
+from pip._internal.exceptions import (
+    NetworkResponseClientError,
+    NetworkResponseServerError,
+)
 from pip._internal.utils.typing import MYPY_CHECK_RUNNING
 
 if MYPY_CHECK_RUNNING:
@@ -30,7 +33,6 @@
 
 def raise_for_status(resp):
     # type: (Response) -> None
-    http_error_msg = u''
     if isinstance(resp.reason, bytes):
         # We attempt to decode utf-8 first because some servers
         # choose to localize their reason strings. If the string
@@ -44,15 +46,9 @@ def raise_for_status(resp):
         reason = resp.reason
 
     if 400 <= resp.status_code < 500:
-        http_error_msg = u'%s Client Error: %s for url: %s' % (
-            resp.status_code, reason, resp.url)
-
+        raise NetworkResponseClientError(resp.status_code, reason, resp.url)
     elif 500 <= resp.status_code < 600:
-        http_error_msg = u'%s Server Error: %s for url: %s' % (
-            resp.status_code, reason, resp.url)
-
-    if http_error_msg:
-        raise NetworkResponseError(http_error_msg, response=resp)
+        raise NetworkResponseServerError(resp.status_code, reason, resp.url)
 
 
 def response_chunks(response, chunk_size=CONTENT_CHUNK_SIZE):

src/pip/_internal/network/xmlrpc.py

@@ -44,9 +44,5 @@ def request(self, host, handler, request_body, verbose=False):
             self.verbose = verbose
             return self.parse_response(response.raw)
         except NetworkResponseError as exc:
-            assert exc.response
-            logger.critical(
-                "HTTP error %s while getting %s",
-                exc.response.status_code, url,
-            )
+            logger.critical("HTTP error %s while getting %s", exc.status, url)
             raise