Envoy to return 400 status instead of dropping request

[tylerschultz]

Cause Envoy to return 400 status code when accessing callback url
when session is expired. This may happen if the user is redirected to
the IdP and takes more time to log in than the session timeout

Fixes #91

Signed-off-by: Tyler Schultz [email protected]

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[margocrawf]

@googlebot I fixed it.

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[margocrawf]

@googlebot I consent.

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CLAs look good, thanks!

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