No way to set isError=True for arbitrary tool result content

[ezyang]

There appears to be no way to manually toggle isError=True for non-text content. For example, you can't return an image with isError=True. I noticed this when implementing a proxy using mcp.tool that recursively dispatches to call_tool. Returning content is fine, but there's no good way to make use of isError, unless you have a single string in which case you can raise an Exception and get isError toggled that way.

[Kludex]

I think the best experience here would be to raise a specific exception that has some content inside, and then we could translate to isError internally.