fix: show custom error for modal state (#240)

Calling a tool that resolves modal state, when there's no such modal
state visible, currently shows this misleading message:

```md
Tool "browser_file_upload" does not handle the modal state.
### Modal state
```

Instead, we should show the error message from the tool implementation.

src/context.ts

@@ -58,6 +58,8 @@ export class Context {
 
   modalStatesMarkdown(): string[] {
     const result: string[] = ['### Modal state'];
+    if (this._modalStates.length === 0)
+      result.push('- There is no modal state present');
     for (const state of this._modalStates) {
       const tool = this.tools.find(tool => tool.clearsModalState === state.type);
       result.push(`- [${state.description}]: can be handled by the "${tool?.schema.name}" tool`);