Harden remote func bootstrapping

Replace bare `source <(curl ...)` calls with a `_bootstrap_source` helper that validates both the download exit code and the presence of a probe function after sourcing. This prevents silent failures when DNS is not yet available in a freshly booted container or when a download fails mid-stream. Applies to build.func, install.func, and alpine-install.func. Also adds retry flags and a connect timeout to curl/wget.

misc/alpine-install.func

@@ -10,8 +10,24 @@ fi
 # must write local failure artifacts instead of talking to the telemetry API
 # (the host is the single telemetry reporter).
 export TELEMETRY_CONTEXT="container"
-source <(curl -fsSL https://raw.githubusercontent.com/community-scripts/ProxmoxVE/main/misc/core.func)
-source <(curl -fsSL https://raw.githubusercontent.com/community-scripts/ProxmoxVE/main/misc/error_handler.func)
+# A freshly booted container may not have DNS up yet. `source <(curl ...)` hides
+# curl's exit code, so a failed download would silently source an empty stream.
+_bootstrap_source() {
+  local url="$1" probe="$2" content
+  content=$(curl -fsSL --connect-timeout 10 --retry 5 --retry-connrefused --retry-delay 2 "$url") || {
+    echo "FATAL: failed to download ${url##*/}" >&2
+    exit 115
+  }
+  source /dev/stdin <<<"$content"
+  declare -f "$probe" >/dev/null 2>&1 || {
+    echo "FATAL: ${url##*/} loaded but incomplete (missing ${probe})" >&2
+    exit 115
+  }
+}
+
+_FUNC_BASE="https://raw.githubusercontent.com/community-scripts/ProxmoxVE/main/misc"
+_bootstrap_source "$_FUNC_BASE/core.func" load_functions
+_bootstrap_source "$_FUNC_BASE/error_handler.func" catch_errors
 load_functions
 catch_errors