refactor: remove unused error reasons

This compose for backport script failure method was a remnant from when
this action used a shell script to perform the git commands. Most
reasons were dead code.

src/backport.ts

@@ -176,9 +176,8 @@ export class Backport {
               this.config.pwd,
             );
           } catch (error) {
-            const message = this.composeMessageForBackportScriptFailure(
+            const message = this.composeMessageForCheckoutFailure(
               target,
-              3,
               baseref,
               headref,
               branchname,
@@ -197,9 +196,8 @@ export class Backport {
           try {
             await this.git.cherryPick(commitShasToCherryPick, this.config.pwd);
           } catch (error) {
-            const message = this.composeMessageForBackportScriptFailure(
+            const message = this.composeMessageForCherryPickFailure(
               target,
-              4,
               baseref,
               headref,
               branchname,
@@ -380,42 +378,60 @@ export class Backport {
                   Please ensure that this Github repo has a branch named \`${target}\`.`;
   }
 
-  private composeMessageForBackportScriptFailure(
+  private composeMessageForCheckoutFailure(
     target: string,
-    exitcode: number,
     baseref: string,
     headref: string,
     branchname: string,
   ): string {
-    const reasons: { [key: number]: string } = {
-      1: "due to an unknown script error",
-      2: "because it was unable to create/access the git worktree directory",
-      3: "because it was unable to create a new branch",
-      4: "because it was unable to cherry-pick the commit(s)",
-      5: "because 1 or more of the commits are not available",
-      6: "because 1 or more of the commits are not available",
-    };
-    const reason = reasons[exitcode] ?? "due to an unknown script error";
-
-    const suggestion =
-      exitcode <= 4
-        ? dedent`\`\`\`bash
-                git fetch origin ${target}
-                git worktree add -d .worktree/${branchname} origin/${target}
-                cd .worktree/${branchname}
-                git checkout -b ${branchname}
-                ancref=$(git merge-base ${baseref} ${headref})
-                git cherry-pick -x $ancref..${headref}
-                \`\`\``
-        : dedent`Note that rebase and squash merges are not supported at this time.
-                For more information see https://github.com/korthout/backport-action/issues/46.`;
+    const reason = "because it was unable to create a new branch";
+    const suggestion = this.composeSuggestion(
+      target,
+      branchname,
+      baseref,
+      headref,
+    );
+    return dedent`Backport failed for \`${target}\`, ${reason}.
 
+                  Please cherry-pick the changes locally.
+                  ${suggestion}`;
+  }
+
+  private composeMessageForCherryPickFailure(
+    target: string,
+    baseref: string,
+    headref: string,
+    branchname: string,
+  ): string {
+    const reason = "because it was unable to cherry-pick the commit(s)";
+    const suggestion = this.composeSuggestion(
+      target,
+      branchname,
+      baseref,
+      headref,
+    );
     return dedent`Backport failed for \`${target}\`, ${reason}.
 
                   Please cherry-pick the changes locally.
                   ${suggestion}`;
   }
 
+  private composeSuggestion(
+    target: string,
+    branchname: string,
+    baseref: string,
+    headref: string,
+  ) {
+    return dedent`\`\`\`bash
+      git fetch origin ${target}
+      git worktree add -d .worktree/${branchname} origin/${target}
+      cd .worktree/${branchname}
+      git checkout -b ${branchname}
+      ancref=$(git merge-base ${baseref} ${headref})
+      git cherry-pick -x $ancref..${headref}
+      \`\`\``;
+  }
+
   private composeMessageForGitPushFailure(
     target: string,
     exitcode: number,