[2.7] bpo-31351: Set return code in ensurepip when pip fails (GH-3734)

Previously ensurepip would always report success, even if the
pip installation failed.

(cherry picked from commit 9adda0cdf89432386b7a04444a6199b580d287a1)
diff --git a/Lib/ensurepip/_uninstall.py b/Lib/ensurepip/_uninstall.py
index 750365e..b257904 100644
--- a/Lib/ensurepip/_uninstall.py
+++ b/Lib/ensurepip/_uninstall.py
@@ -2,6 +2,7 @@
 
 import argparse
 import ensurepip
+import sys
 
 
 def _main(argv=None):
@@ -23,8 +24,8 @@
 
     args = parser.parse_args(argv)
 
-    ensurepip._uninstall_helper(verbosity=args.verbosity)
+    return ensurepip._uninstall_helper(verbosity=args.verbosity)
 
 
 if __name__ == "__main__":
-    _main()
+    sys.exit(_main())