fix: handle failed global subgraph blueprint loading gracefully (#9063)

## Summary

Fix "Failed to load subgraph blueprints Error: [ASSERT] Workflow content
should be loaded" error occurring on cloud.

## Changes

- **What**: `getGlobalSubgraphData` now throws on API failure instead of
returning empty string, global blueprint data is validated before
loading, and individual global blueprint errors are properly propagated
to the toast/console reporting instead of being silently swallowed.

## Review Focus

Two root causes were fixed:
1. `getGlobalSubgraphData` returned `""` on failure — this empty string
was set as `originalContent`, which is falsy, triggering the assertion
in `ComfyWorkflow.load()`.
2. `loadInstalledBlueprints` used an internal `Promise.allSettled` whose
results were discarded, so individual global blueprint failures never
reached the error reporting in `fetchSubgraphs`.

Fixes COM-15199

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9063-fix-handle-failed-global-subgraph-blueprint-loading-gracefully-30e6d73d3650818d9cc8ecf81cd0264e)
by [Unito](https://www.unito.io)

src/scripts/api.ts

@@ -1181,9 +1181,16 @@ export class ComfyApi extends EventTarget {
 
   async getGlobalSubgraphData(id: string): Promise<string> {
     const resp = await api.fetchApi('/global_subgraphs/' + id)
-    if (resp.status !== 200) return ''
+    if (resp.status !== 200) {
+      throw new Error(
+        `Failed to fetch global subgraph '${id}': ${resp.status} ${resp.statusText}`
+      )
+    }
     const subgraph: GlobalSubgraphData = await resp.json()
-    return subgraph?.data ?? ''
+    if (!subgraph?.data) {
+      throw new Error(`Global subgraph '${id}' returned empty data`)
+    }
+    return subgraph.data as string
   }
   async getGlobalSubgraphs(): Promise<Record<string, GlobalSubgraphData>> {
     const resp = await api.fetchApi('/global_subgraphs')