Improve error handling and relative file path support for extension & prompts #143
Description
Current state
Some user provided values are required (and some values are optional) when using the MCP-hosted prompts as slash commands in VS Code, where using an invalid or unresolved (relative) path returns a cryptic MCP protocol error, but only after the user has provided all required and optional inputs.
Desired state
A valid, workspace-root-relative path should always be resolved when used as a prompt input, rather that throwing an error due to file not found. For any error that is thrown to or by the slash command handler for a given MCP-hosted prompts, such errors should fail early, and fail with useful error messages.