Inform users when vcs's are not in the path.

news/4461.bugfix

@@ -0,0 +1,4 @@
+This fixes an issue where when someone who tries to use git
+with pip but pip can't because git is not in the path
+environment variable.  This clarifies the error given to
+suggest to the user what might be wrong.

pip/vcs/__init__.py

@@ -327,7 +327,10 @@ def run_command(self, cmd, show_stdout=True, cwd=None,
             # errno.ENOENT = no such file or directory
             # In other words, the VCS executable isn't available
             if e.errno == errno.ENOENT:
-                raise BadCommand('Cannot find command %r' % self.name)
+                raise BadCommand(
+                    'Cannot find command %r - do you have '
+                    '%r installed and in your '
+                    'PATH?' % (self.name, self.name))
             else:
                 raise  # re-raise exception if a different error occurred