WorkspaceStarter: fix + cleanup error handling

[geropl]

Description

This PR does three things:

1. does away with the feature flag workspace_start_check_pending_first
2. fixes a bug that failed to map ScmStartError to StartInstanceError
3. merges the two cascading try...catch block into a single one

To be more safe while rolling this out, we also keep the old code around, and have the feature flag workspace_start_old to switch back to it should be notice anything odd.

:information_source: The PR is best reviewed a) commit-per-commit and b) with "split view"+"whitespace hidden":

Related Issue(s)

Fixes ENT-178

How to test

Documentation

Preview status

Gitpod was successfully deployed to your preview environment.

• 🏷️ Name - gpl-178-scm-error
• 🔗 URL - gpl-178-scm-error.preview.gitpod-dev.com/workspaces.
• 📚 Documentation - See our internal documentation for information on how to interact with your preview environment.
• 📦 Version - gpl-178-scm-error-gha.25431
• 🗒️ Logs - GCP Logs Explorer

Build Options

Build

• [ ] /werft with-werft Run the build with werft instead of GHA
• [ ] leeway-no-cache
• [ ] /werft no-test Run Leeway with --dont-test

Publish

• [ ] /werft publish-to-npm
• [ ] /werft publish-to-jb-marketplace

Installer

• [ ] analytics=segment
• [ ] with-dedicated-emulation
• [ ] workspace-feature-flags Add desired feature flags to the end of the line above, space separated

Preview Environment / Integration Tests

• [ ] /werft with-local-preview If enabled this will build install/preview
• [x] /werft with-preview
• [ ] /werft with-large-vm
• [x] /werft with-gce-vm If enabled this will create the environment on GCE infra
• [x] /werft preemptible Saves cost. Untick this only if you're really sure you need a non-preemtible machine.
• [ ] with-integration-tests=all Valid options are all, workspace, webapp, ide, jetbrains, vscode, ssh. If enabled, with-preview and with-large-vm will be enabled.
• [ ] with-monitoring

/hold