fix(backport): comment error when target not found

When the target cannot be found on remote, then the git fetch fails
with: (example for `maintenance/8.1`)
```
fatal: couldn't find remote ref maintenance/8.1
'git fetch origin maintenance/8.1' failed with exit code 128
```

We can improve this by throwing a special error and handling it
explicitly by commenting on the original pull request about this
situation. We can also help the user figure out how to resolve the
problem by making it clear that a branch with that name should exist.

(cherry picked from commit f16f704)

src/backport.ts

@@ -106,7 +106,23 @@ export class Backport {
         const target = match[1];
         console.log(`Found target in label: ${target}`);
 
-        await git.fetch(target, this.config.pwd, 1);
+        try {
+          await git.fetch(target, this.config.pwd, 1);
+        } catch (error) {
+          if (error instanceof git.GitRefNotFoundError) {
+            const message = this.composeMessageForFetchTargetFailure(error.ref);
+            console.error(message);
+            await this.github.createComment({
+              owner,
+              repo,
+              issue_number: pull_number,
+              body: message,
+            });
+            continue;
+          } else {
+            throw error;
+          }
+        }
 
         try {
           const branchname = `backport-${pull_number}-to-${target}`;
@@ -243,6 +259,11 @@ export class Backport {
     return { title, body };
   }
 
+  private composeMessageForFetchTargetFailure(target: string) {
+    return dedent`Backport failed for \`${target}\`: couldn't find remote ref \`${target}\`.
+                  Please ensure that this Github repo has a branch named \`${target}\`.`;
+  }
+
   private composeMessageForBackportScriptFailure(
     target: string,
     exitcode: number,

src/git.ts

@@ -1,19 +1,34 @@
 import execa from "execa";
 
+export class GitRefNotFoundError extends Error {
+  ref: string;
+  constructor(message: string, ref: string) {
+    super(message);
+    this.ref = ref;
+  }
+}
+
 /**
  * Fetches a ref from origin
  *
  * @param ref the sha, branchname, etc to fetch
  * @param pwd the root of the git repository
  * @param depth the number of commits to fetch
+ * @throws GitRefNotFoundError when ref not found
+ * @throws Error for any other non-zero exit code
  */
 export async function fetch(ref: string, pwd: string, depth: number) {
   const { exitCode } = await git(
     "fetch",
     [`--depth=${depth}`, "origin", ref],
     pwd
   );
-  if (exitCode !== 0) {
+  if (exitCode === 128) {
+    throw new GitRefNotFoundError(
+      `Expected to fetch '${ref}', but couldn't find it`,
+      ref
+    );
+  } else if (exitCode !== 0) {
     throw new Error(
       `'git fetch origin ${ref}' failed with exit code ${exitCode}`
     );