chore(mcp): Handle all Backstage errors, both built-in and custom

[johnmcollier]

Hey, I just made a Pull Request!

Follow up PR to https://github.com/backstage/backstage/pull/31847

This PR updates handleErrors.ts in mcp-actions-backend to handle any custom Backstage error, rather than just the built-in errors in @backstage/errors. This will be useful for any MCP tools that interact with plugins / custom integrations that have their own error set outside of the standard errors provided.

I've also added a couple basic test cases to cover the logic in handleErrors.ts

:heavy_check_mark: Checklist

• [x] A changeset describing the change and affected packages. (more info)
• [x] Added or updated documentation
• [x] Tests for new functionality and regression tests for bug fixes
• [ ] Screenshots attached (for UI changes)
• [x] All your commits have a Signed-off-by line in the message. (more info)

[backstage-goalie[bot]]

[!IMPORTANT] This PR includes changes that affect public-facing API. Please ensure you are adding/updating documentation for new features or behavior.

Changed Packages

Package Name	Package Path	Changeset Bump	Current Version
@backstage/errors	packages/errors	minor	v1.2.7
@backstage/plugin-mcp-actions-backend	plugins/mcp-actions-backend	patch	v0.1.6-next.1

[johnmcollier]

Example of custom error being thrown in an MCP action: