A failed update install says the install failed, not the check

installUpdate, and a failed channel or source switch, all fell back to "The
update check failed." Each now names what failed. A check that runs after a
successful switch keeps its own message.

Fixes #5387.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/views/SyncView.vue

@@ -156,7 +156,7 @@ async function switchChannel(next: UpdateChannel) {
     checkedOnce.value = false;
     await checkUpdates();
   } catch (e) {
-    updateError.value = errorMessage(e, "The update check failed.");
+    updateError.value = errorMessage(e, "The update channel couldn't be changed.");
   }
 }
 
@@ -170,7 +170,7 @@ async function switchSource(next: string | null) {
     checkedOnce.value = false;
     await checkUpdates();
   } catch (e) {
-    updateError.value = errorMessage(e, "The update check failed.");
+    updateError.value = errorMessage(e, "The update source couldn't be changed.");
   }
 }
 
@@ -182,7 +182,7 @@ async function installUpdate() {
     // means it failed.
     await updateBridge.install();
   } catch (e) {
-    updateError.value = errorMessage(e, "The update check failed.");
+    updateError.value = errorMessage(e, "The update couldn't be installed.");
   } finally {
     installing.value = false;
   }