Closes SRCH-524

Slack Context

Bitbucket Server (v5.1) added the ability to decline and delete pull requests - this isn't replicated across other supported codehosts such as:

• GITHUB
• GITLAB
• BITBUCKET CLOUD
  e.t.c

Due to this, whenever a pull request is deleted, Batch Changes services (like Syncer & Reconciler) keep retrying whatever operation they're running, thereby spamming the codehost in the process.
In this PR, I ensured whenever we encounter certain status code from a HTTP request to a codehost, it returns a NonRetryable error to reduce spamming.

The logic for handling non-existent pull requests lived here, however that condition was never met because httpError did not fulfil the Unwrap interface therefore errors.As couldn't unwrap the underlying error.

Test plan

Create a test batch change targeting a bitbucket server codehost. Once the pull request has been created, delete the pull request.

Once the changeset has been synced on Sourcegraph, the status of the changeset will be updated toCLOSED.

name: testbbdecline2
description: Add Hello World to READMEs

# Find the first 100 search results for README files.
# This could yield less than 100 repos/workspaces if some repos contain multiple READMEs.
on:
  - repositoriesMatchingQuery: repo:^bitbucket\.sgdev\.org/BABU/(delve|compose)$ 

# In each repository, run this command. Each repository's resulting diff is captured.
steps:
  - run: IFS=$'\n'; echo Hello World | tee -a $(find -name README.md)
    container: ubuntu:18.04

# Describe the changeset (e.g., GitHub pull request) you want for each repository.
changesetTemplate:
  title: Hello World
  body: My first batch change!
  commit:
    message: Append Hello World to all README.md files
  # Optional: Push the commit to a branch named after this batch change by default.
  branch: ${{ batch_change.name }}

Changelog

• Disallow auto-retry when a changeset is deleted.