Fix function build errors not rendering actual failure reason

[davejcameron]

Summary

• When a Shopify Function build command fails (e.g. cargo/rust not installed), app-event-watcher only wrote error.message ("Failed to build function.") to stderr, completely hiding the actual failure reason
• The detail was stored in error.tryMessage (e.g. "Command failed with exit code 127: cargo build --release") but never rendered
• Now writes tryMessage to stderr when present, so users see why their build actually failed

Test plan

• Added test in app-event-watcher.test.ts verifying tryMessage content is written to stderr
• Added tests in extension.test.ts verifying tryMessage is preserved when wrapping ExternalError and generic errors
• All existing tests pass
• Lint and typecheck pass

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Improves the developer experience when Shopify Function builds fail during dev by ensuring the underlying failure reason (stored in tryMessage) is written to stderr instead of only showing a generic error message.

Changes:

• Print error.tryMessage to stderr in AppEventWatcher when present, to surface the actual build failure reason.
• Add a dev watcher test to verify tryMessage content is included in stderr output.
• Add build service tests to verify build command failure details are preserved in AbortError.tryMessage.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
packages/app/src/cli/services/dev/app-events/app-event-watcher.ts	Writes tryMessage to stderr on non-esbuild build errors.
packages/app/src/cli/services/dev/app-events/app-event-watcher.test.ts	Adds coverage asserting tryMessage details appear in stderr output.
packages/app/src/cli/services/build/extension.test.ts	Adds coverage asserting build command failures propagate details via tryMessage.

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[Copilot]

When printing error.message and then error.tryMessage, the two writes are concatenated with no separator, so users may see a single glued string (e.g., Failed to build function.Command failed ...). Consider writing a newline (or \n\n) between them, or only adding a separator if error.message doesn’t already end with one, so the output is readable.

Suggested change

	this.options.stderr.write(
	typeof error.tryMessage === 'string' ? error.tryMessage : String(error.tryMessage),
	)
	const tryMessage =
	typeof error.tryMessage === 'string' ? error.tryMessage : String(error.tryMessage)
	if (!String(error.message).endsWith('\n')) {
	this.options.stderr.write('\n')
	}
	this.options.stderr.write(tryMessage)

[Copilot]

AbortError.tryMessage is typed as a TokenItem | null (not necessarily a plain string). Using String(error.tryMessage) can yield [object Object] for tokenized messages. Since this file already imports from @shopify/cli-kit/node/output, consider using itemToString(error.tryMessage) (or equivalent) to reliably render token items to a human-readable string before writing to stderr.