Improved Mechanism for Propagating Tool Failure

[eiriktsarpalis]

Originally posted by @PederHP in PederHP/mcpdotnet#43.

Tool failure (using IsError = true) is currently not possible to return from the Tool mapping layer. This could be done via:

1. Enforcing all Tools return a ToolResult type which has an IsError field. This is not desirable as it removes the ability to expose any method as tool.

2. A new Exception (ie McpToolException) which is caught in the handler and then returns IsError true and an error message provided in the exception. This would allow developers a way to hook into the MCP protocol without enforcing specific return types on their method.