BuildStream hangs on certain network error conditions

Extract from Gitlab
```
[02:01:52][--:--:--][f40511d0][ pull:flatpak-images/platform-arch-libs-debug.bst] WARNING Could not pull from remote https://cache.freedesktop-sdk.io:11002/: Failed to fetch directory tree f9533479dcd7eeea8305464889d309282b6584da8cca5ad4ea402342230b6fc5: UNAVAILABLE: Socket closed
[02:01:52][--:--:--][ ][ main:core activity ] BUG buildbox-casd died while the pipeline was active.
[02:01:52][--:--:--][4c894216][ main:flatpak-images/glxinfo.bst ] STATUS Pull terminating
[02:01:52][--:--:--][4c099039][ main:flatpak-images/mesa-debug.bst ] STATUS Pull terminating
[02:01:52][--:--:--][c68c0db5][ main:flatpak-images/mesa.bst ] STATUS Pull terminating
[02:01:52][--:--:--][f40511d0][ main:flatpak-images/platform-arch-libs-debug.bst] STATUS Pull terminating
[02:01:52][--:--:--][96520b61][ main:flatpak-images/platform-arch-libs.bst] STATUS Pull terminating
[02:01:52][--:--:--][60954554][ main:flatpak-images/platform-ffmpeg-full.bst] STATUS Pull terminating
[02:01:52][--:--:--][2fa11719][ main:flatpak-images/platform-locale.bst] STATUS Pull terminating
[02:01:52][--:--:--][315caa33][ main:flatpak-images/platform-vaapi-intel.bst] STATUS Pull terminating
[02:01:52][--:--:--][98e5eb5b][ main:flatpak-images/platform.bst ] STATUS Pull terminating
[02:01:52][--:--:--][312c5b4a][ main:flatpak-images/glxinfo-debug.bst] STATUS Push terminating
[02:01:52][--:--:--][f63306cf][ main:sdk-image.bst ] STATUS Pull terminating
[02:01:52][--:--:--][60954554][ main:flatpak-images/platform-ffmpeg-full.bst] INFO Job terminated
BUG: Message handling out of sync, unable to retrieve failure message for element flatpak-images/platform-ffmpeg-full.bst
[02:01:52][--:--:--][f40511d0][ main:flatpak-images/platform-arch-libs-debug.bst] INFO Job terminated
BUG: Message handling out of sync, unable to retrieve failure message for element flatpak-images/platform-arch-libs-debug.bst
[02:01:52][--:--:--][96520b61][ main:flatpak-images/platform-arch-libs.bst] INFO Job terminated
BUG: Message handling out of sync, unable to retrieve failure message for element flatpak-images/platform-arch-libs.bst
[02:01:52][--:--:--][f63306cf][ main:sdk-image.bst ] INFO Job terminated
BUG: Message handling out of sync, unable to retrieve failure message for element sdk-image.bst
```

The report provides no source file, test, or entry point to begin from; it only includes a BuildStream log showing a socket-closed error followed by buildbox-casd termination and out-of-sync failure messages. Reproduce the network-error condition and determine what behavior should replace the hang, including whether each terminated job reports its failure correctly.

Written by the indexing model from the issue text.