[3.6] bpo-31351: Set return code in ensurepip when pip fails (GH-3626)

[miss-islington]

Previously ensurepip would always report success, even if the
pip installation failed.
(cherry picked from commit 9adda0c)

https://bugs.python.org/issue31351

[the-knights-who-say-ni]

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[miss-islington]

Thanks, @Mariatta!