Review `McpError` use

[chemicL]

While working on #48 I noticed McpError is used quite liberally, even in situations which don't have to do with the JSON-RPC layer.

For one instance, adding a new Tool to an existing server can yield an McpError which really cannot be propagated over the wire since those interactions are not initiated by the peer, but are an internal server change. I believe for such cases a different Exception type should be used.