**What happens?**:
When running a build of the train-dev environment, error was encountered while preparing the git repository mirror, but the build did not fail:
```
Preparing Gerrit repository mirror. This will take a while
Cloning https://gerrit.wikimedia.org/r//mediawiki/core.git into mediawiki/core.git
Cloning https://gerrit.wikimedia.org/r//mediawiki/tools/release.git into mediawiki/tools/release.git
Cloning https://gerrit.wikimedia.org/r//mediawiki/tools/scap.git into mediawiki/tools/scap.git
Cloning https://gerrit.wikimedia.org/r//mediawiki/extensions.git into mediawiki/extensions.git
Cloning https://gerrit.wikimedia.org/r//mediawiki/services/function-schemata.git into mediawiki/services/function-schemata.git
Cloning https://gerrit.wikimedia.org/r//mediawiki/skins.git into mediawiki/skins.git
Cloning https://gerrit.wikimedia.org/r//mediawiki/vendor.git into mediawiki/vendor.git
Cloning https://gerrit.wikimedia.org/r//operations/mediawiki-config.git into operations/mediawiki-config.git
Cloning https://gerrit.wikimedia.org/r//operations/mediawiki-config/fonts.git into operations/mediawiki-config/fonts.git
Cloning https://gerrit.wikimedia.org/r//operations/deployment-charts.git into operations/deployment-charts.git
Cloning https://gerrit.wikimedia.org/r//VisualEditor/VisualEditor.git into VisualEditor/VisualEditor.git
Cloning https://gerrit.wikimedia.org/r//wikibase/javascript-api.git into wikibase/javascript-api.git
Cloning https://gerrit.wikimedia.org/r//wikibase/termbox.git into wikibase/termbox.git
Cloning https://gerrit.wikimedia.org/r//data-values/value-view.git into data-values/value-view.git
Cloning https://gerrit.wikimedia.org/r//wikimedia/portals/deploy.git into wikimedia/portals/deploy.git
Cloning https://phabricator.wikimedia.org/source//wikibase-data-model.git into wikibase-data-model.git
Cloning https://phabricator.wikimedia.org/source//wikibase-serialization.git into wikibase-serialization.git
Cloning https://phabricator.wikimedia.org/source//wikibase-termbox.git into wikibase-termbox.git
Cloning https://phabricator.wikimedia.org/source//datavalues-javascript.git into datavalues-javascript.git
Processing all extensions
Cloning https://gerrit.wikimedia.org/r//mediawiki/extensions/3D.git into mediawiki/extensions/3D.git
Cloning https://gerrit.wikimedia.org/r//mediawiki/extensions/AControlImageLink.git into mediawiki/extensions/AControlImageLink.git
Cloning https://gerrit.wikimedia.org/r//mediawiki/extensions/AJAXPoll.git into mediawiki/extensions/AJAXPoll.git
.
.
.
Cloning https://gerrit.wikimedia.org/r//mediawiki/extensions/SafeDelete.git into mediawiki/extensions/SafeDelete.git
Cloning https://gerrit.wikimedia.org/r//mediawiki/extensions/SandboxLink.git into mediawiki/extensions/SandboxLink.git
git remote update failed!
stdout:Fetching origin
stderr:POST git-upload-pack (128 bytes)
POST git-upload-pack (490 bytes)
error: RPC failed; curl 56 LibreSSL SSL_read: SSL_ERROR_SYSCALL, errno 60
error: 34828 bytes of body are still expected
fetch-pack: unexpected disconnect while reading sideband packet
fatal: early EOF
fatal: fetch-pack: invalid index-pack output
error: Could not fetch origin
Cloning https://gerrit.wikimedia.org/r//mediawiki/extensions/Sarcasm.git into mediawiki/extensions/Sarcasm.git
Cloning https://gerrit.wikimedia.org/r//mediawiki/extensions/SaveSpinner.git into mediawiki/extensions/SaveSpinner.git
.
.
.
Cloning https://gerrit.wikimedia.org/r//mediawiki/extensions/WikiLovesMonuments.git into mediawiki/extensions/WikiLovesMonuments.git
Cloning https://gerrit.wikimedia.org/r//mediawiki/extensions/WikiSEO.git into mediawiki/extensions/WikiSEO.git
git remote update failed!
stdout:Fetching origin
stderr:POST git-upload-pack (128 bytes)
error: RPC failed; curl 56 LibreSSL SSL_read: SSL_ERROR_SYSCALL, errno 60
fatal: expected flush after ref listing
error: Could not fetch origin
Cloning https://gerrit.wikimedia.org/r//mediawiki/extensions/WikiShare.git into mediawiki/extensions/WikiShare.git
```
The mirror-repos.sh script is running some commands in parallel to speed up the build.
**What should have happened instead?**:
* The build should fail, or it should re-attempt the failed update
** mirror-repos.sh should ensure all processes have succeeded in order to continue
**Software version (if not a Wikimedia wiki), browser information, screenshots, other information, etc.**: