common-updater-scripts: fix silent error on 404

When TOFU was unable to download the file, there would be no hash
in the fetch log, causing the grep to fail. Since the script
is set to errexit, it would terminate the processing without
any output. Let’s instead print the fetch log.

pkgs/common-updater/scripts/update-source-version

@@ -237,9 +237,13 @@ fi
 if [[ -z "$newHash" ]]; then
     nix-build $systemArg --no-out-link -A "$attr.$sourceKey" 2>"$attr.fetchlog" >/dev/null || true
     # FIXME: use nix-build --hash here once https://github.com/NixOS/nix/issues/1172 is fixed
-    newHash=$(sed '1,/hash mismatch in fixed-output derivation/d' "$attr.fetchlog" | grep --perl-regexp --only-matching 'got: +.+[:-]\K.+')
+    newHash=$(
+        sed '1,/hash mismatch in fixed-output derivation/d' "$attr.fetchlog" \
+        | grep --perl-regexp --only-matching 'got: +.+[:-]\K.+' \
+        || true # handled below
+    )
 
-    if [[ -n "$sri" ]]; then
+    if [[ -n "$newHash" && -n "$sri" ]]; then
         # nix-build preserves the hashing scheme so we can just convert the result to SRI using the old type
         newHash="$(nix --extra-experimental-features nix-command hash to-sri --type "$oldHashAlgo" "$newHash" 2>/dev/null \
             || nix to-sri --type "$oldHashAlgo" "$newHash" 2>/dev/null)" \