Make --require-virtualenv output more info

if not in a virtualenv **and** --user wasn't specified.

Previously, this would print an error even if --user was specified.
I believe the spirit of `--require-virtualenv` is that it was meant to
be used as `require_virtualenv = True` in the pip config file and what
it really seeks to do is to prevent the user from messing with packages
in the system python envvironment. Thus, I'd argue that --user shouldn't
cause an error, because it doesn't mess with the system python.

With this change, output looks like this:

    $ python -m pip install --require-virtualenv foo
    ******************************************************
    Using pip outside a virtualenv and without --user
    is not recommended!

    Doing stuff with the system python's packages can break
    your system.

    Did you mean to use a virtualenv or use --user?
    ******************************************************

Author: msabramo

pip/basecommand.py

@@ -4,6 +4,7 @@
 import logging
 import os
 import sys
+import textwrap
 import traceback
 import optparse
 import warnings
@@ -214,10 +215,20 @@ def main(self, args):
         if options.require_venv:
             # If a venv is required check if it can really be found
             if not running_under_virtualenv():
-                logger.critical(
-                    'Could not find an activated virtualenv (required).'
-                )
-                sys.exit(VIRTUALENV_NOT_FOUND)
+                if not getattr(options, 'use_user_site'):
+                    logger.critical(
+                        textwrap.dedent("""\
+                        ******************************************************
+                        Using pip outside a virtualenv and without --user
+                        is not recommended!
+
+                        Doing stuff with the system python's packages can break
+                        your system.
+
+                        Did you mean to use a virtualenv or use --user?
+                        ******************************************************""")
+                    )
+                    sys.exit(VIRTUALENV_NOT_FOUND)
 
         # Check if we're using the latest version of pip available
         if (not options.disable_pip_version_check and not