fix: address verified review findings (pulls-1)

Signed-off-by: bircni <bircni@icloud.com>

internal/services/errors.go

@@ -159,6 +159,9 @@ var (
 	unknownUser = regexp.MustCompile(`user does not exist \[uid: \d+, name: ([^\]]+)\]`)
 	// pullTargets finds the branches in "pull request already exists for these targets [... head_branch: x, base_branch: y]".
 	pullTargets = regexp.MustCompile(`head_branch: ([^,\]]+), base_branch: ([^,\]]+)`)
+	// missingBranch finds the branch in the 404 messages `ref "x" does not exist` (create pull
+	// request) and `new base 'x' not exist` (edit pull request).
+	missingBranch = regexp.MustCompile(`^(?:ref "([^"]+)" does not exist|new base '([^']+)' not exist)$`)
 	// wikiTitle finds the page in "wiki page already exists [title: Home.md]".
 	wikiTitle = regexp.MustCompile(`\[title: (.+?)(?:\.md)?\]`)
 )
@@ -186,6 +189,9 @@ func Classify(err error, kind, key, scope string) error {
 		}
 		return conflict
 	}
+	if match := missingBranch.FindStringSubmatch(apiErr.Message); apiErr.Status == http.StatusNotFound && match != nil {
+		return &NotFoundError{Kind: "branch", Key: strconv.Quote(cmp.Or(match[1], match[2])), Scope: scope, Err: err}
+	}
 	if apiErr.Status == http.StatusNotFound && kind != "" {
 		return &NotFoundError{Kind: kind, Key: key, Scope: scope, Err: err}
 	}