Tool execution errors should be returned as tool results with isError: true and fix error response formatting

[kpschaper]

1. Tool execution errors should be returned as tool results with isError: true (mcp/server.go)
Currently, when wrong input parameters are passed to a tool the server would return a JSON-RPC error and hang (waiting for response), this does not allow the LLM to self-correct and retry with adjusted parameters.
Reference; https://modelcontextprotocol.io/specification/2025-11-25/server/tools#error-handling

2. Fix error response formatting (client/response):
The OData v2 spec specifies an error to return an object with both a code and a message. The code is optional while the message is mandatory. The current implementation checks for a Code while it should check for a Message.

Before: the error returned looks like (because a fallback is used):

HTTP 400: {"error":{"code":"","message":{"lang":"en-GB","value":"Unknown function 'contains' at position 0."}}}

After:

OData error (HTTP 400): Unknown function 'contains' at position 0.