No error is shown when PR does not exist

[dmitrivMS]

Testing microsoft/vscode#274805

Repro Steps

1. Enlist and load vscode repo
2. From browser open a PR with number that does not exist, e.g. vscode-insiders://github.vscode-pull-request-github/checkout-pull-request?%7B%22owner%22%3A%22microsoft%22%2C%22repo%22%3A%22vscode%22%2C%22pullRequestNumber%22%3A987654321%7D

Actual Result
Nothing happens (you can look in the Output window and see the error, but you don't know you have to look there)

Expected Result
Error message that PR does not exist.

Output Window Contents
2025-11-04 14:11:04.300 [error] [GitHubRepository+0] Error querying GraphQL API (PullRequest): GraphQL error: Could not resolve to a PullRequest with the number of 987654321..
2025-11-04 14:11:04.300 [error] [GitHubRepository+0] Unable to fetch PR: Error: GraphQL error: Could not resolve to a PullRequest with the number of 987654321.
2025-11-04 14:11:04.300 [warning] [UriHandler] Pull request #987654321 not found for checkout.