gio: Update bad cert error in accept-certificate and GTlsError docs

This makes the documented error returned when the cert is rejected
consistent with both self and reality.
11 jobs from wip/mjog/accept-certificate-docs in 18 minutes and 47 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #183160
cross-android_api21_arm64

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passed #183161
cross-android_api28_arm64

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passed #183162
cross-mingw64

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passed #183184
fedora-autotools-x86_64

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passed #183183
fedora-x86_64

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passed #183165
freebsd-11
freebsd-11-x86_64

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passed #183163
win32
msys2-mingw32

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passed #183164
win32
vs2017-x64

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failed #183159
fedora-autotools-x86_64

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failed #183158
fedora-x86_64

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  Coverage
passed #183166
coverage

00:02:11

75.5%