Improve reporting for dependency-graph failure

The previous message was assuming a permissions issue, and was not
including the underlying error message in the response.

src/dependency-graph.ts

@@ -78,12 +78,7 @@ async function submitDependencyGraphs(dependencyGraphFiles: string[]): Promise<v
             await submitDependencyGraphFile(jsonFile)
         } catch (error) {
             if (error instanceof RequestError) {
-                const relativeJsonFile = getRelativePathFromWorkspace(jsonFile)
+                core.warning(buildWarningMessage(jsonFile, error))
-                core.warning(
-                    `Failed to submit dependency graph ${relativeJsonFile}.\n` +
-                        "Please ensure that the 'contents: write' permission is available for the workflow job.\n" +
-                        "Note that this permission is never available for a 'pull_request' trigger from a repository fork."
-                )
             } else {
                 throw error
             }
@@ -91,6 +86,18 @@ async function submitDependencyGraphs(dependencyGraphFiles: string[]): Promise<v
     }
 }
 
+function buildWarningMessage(jsonFile: string, error: RequestError): string {
+    const relativeJsonFile = getRelativePathFromWorkspace(jsonFile)
+    const mainWarning = `Failed to submit dependency graph ${relativeJsonFile}.\n${String(error)}`
+    if (error.message === 'Resource not accessible by integration') {
+        return `${mainWarning}
+Please ensure that the 'contents: write' permission is available for the workflow job.
+Note that this permission is never available for a 'pull_request' trigger from a repository fork.
+        `
+    }
+    return mainWarning
+}
+
 async function submitDependencyGraphFile(jsonFile: string): Promise<void> {
     const octokit = getOctokit()
     const jsonContent = fs.readFileSync(jsonFile, 'utf8')