Improve error message when workflow file doesn't exist

lib/workflow.js

@@ -213,7 +213,16 @@ exports.formatWorkflowCause = formatWorkflowCause;
 async function getWorkflow() {
     const relativePath = await getWorkflowPath();
     const absolutePath = path.join((0, util_1.getRequiredEnvParam)("GITHUB_WORKSPACE"), relativePath);
-    return yaml.load(fs.readFileSync(absolutePath, "utf-8"));
+    try {
+        return yaml.load(fs.readFileSync(absolutePath, "utf-8"));
+    }
+    catch (e) {
+        if (e instanceof Error && e["code"] === "ENOENT") {
+            throw new Error(`Unable to load code scanning workflow from ${absolutePath}. This can happen if the currently ` +
+                "running workflow checks out a branch that doesn't contain the corresponding workflow file.");
+        }
+        throw e;
+    }
 }
 exports.getWorkflow = getWorkflow;
 /**

src/workflow.ts

@@ -259,7 +259,17 @@ export async function getWorkflow(): Promise<Workflow> {
     relativePath
   );
 
-  return yaml.load(fs.readFileSync(absolutePath, "utf-8")) as Workflow;
+  try {
+    return yaml.load(fs.readFileSync(absolutePath, "utf-8")) as Workflow;
+  } catch (e) {
+    if (e instanceof Error && e["code"] === "ENOENT") {
+      throw new Error(
+        `Unable to load code scanning workflow from ${absolutePath}. This can happen if the currently ` +
+          "running workflow checks out a branch that doesn't contain the corresponding workflow file."
+      );
+    }
+    throw e;
+  }
 }
 
 /**