pull_requests: return 404 on invalid repo or PR (bug 2002104) r=zeid

This PR updates the base GitHub utility to raise any HTTP error as exceptions, so they bubble up to higher layers.

It then captures those in the Pull Request UI view to detect non-existent PRs, and return a cleaner 404 with appropriate message. It also does the same for non-existent Repos.

The Jinja template is also updated to include the exception message, if present, to present more context to the user.

Pull request: #741

Author: shtrom

src/lando/ui/pull_requests.py

@@ -3,6 +3,7 @@
 from django.core.handlers.wsgi import WSGIRequest
 from django.http import Http404
 from django.template.response import TemplateResponse
+from requests import HTTPError
 
 from lando.main.models import Repo
 from lando.main.models.landing_job import get_jobs_for_pull
@@ -19,13 +20,27 @@ def get(
         self, request: WSGIRequest, repo_name: str, number: int, *args, **kwargs
     ) -> TemplateResponse:
         """Handle the GET request for the pull request view."""
-        target_repo = Repo.objects.get(name=repo_name)
+        try:
+            target_repo = Repo.objects.get(name=repo_name)
+        except Repo.DoesNotExist:
+            raise Http404(f"Repository {repo_name} doesn't exist.")
 
         if not target_repo.pr_enabled:
-            raise Http404("Pull Requests are not supported for this repository.")
+            raise Http404(
+                f"Pull Requests are not supported for repository {repo_name}."
+            )
 
         client = GitHubAPIClient(target_repo.url)
-        pull_request = client.build_pull_request(number)
+
+        try:
+            pull_request = client.build_pull_request(number)
+        except HTTPError as e:
+            if e.response.status_code == 404:
+                raise Http404(
+                    f"Pull request {repo_name}#{number} doesn't exist."
+                ) from e
+            raise e
+
         landing_jobs = get_jobs_for_pull(target_repo, number)
 
         context = {

src/lando/utils/github.py

@@ -184,6 +184,7 @@ def _repo_get(self, subpath: str, *args, **kwargs) -> dict | list:
 
     def _get(self, path: str, *args, **kwargs) -> dict | list | str | None:
         result = self._api.get(path, *args, **kwargs)
+        result.raise_for_status()
         content_type = result.headers["content-type"]
         if content_type == "application/json; charset=utf-8":
             return result.json()