`analyze_files` should have better error when encountering a missing path.

[gspencergoog]

The Gemini CLI agent ran the analyze_files tool with an invalid path and it gave this error.

 ╭───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╮
 │ x  analyze_files (dart MCP Server) {"roots":[{"root":"file:///usr/L/g/h/g/c/g/packages/spikes/json_client"}]}         │
 │                                                                                                                           │
 │    MCP tool 'analyze_files' reported tool error for function call:                                                        │
 │    {"name":"analyze_files","args":{"roots":[{"root":"file:///usr/L/g/h/g/c/g/packages/spikes/json_client"}]}} with    │
 │    response: [{"functionResponse":{"name":"analyze_files","response":{"error":{"content":[{"type":"text","text":"Null     │
 │    check operator used on a null value\n#0      DartAnalyzerSupport._analyzeFiles                                         │
 │    (package:dart_mcp_server/src/mixins/analyzer.dart:277)\n<asynchronous suspension>\n#1      ToolsSupport._callTool      │
 │    (package:dart_mcp/src/server/tools_support.dart:113)\n<asynchronous suspension>\n#2      Server._handleSingleRequest   │
 │    (package:json_rpc_2/src/server.dart:212)\n<asynchronous suspension>\n#3      Server._handleRequest                     │
 │    (package:json_rpc_2/src/server.dart:187)\n<asynchronous suspension>\n"}],"isError":true}}}}]                           │
 ╰───────────────────────────────────────────────────────────────────────────────────────────────────────

I would have expected it to generate something like:

MCP tool 'analyze_files' reported tool error for function call: 
ERROR: Supplied root file:///usr/L/g/h/g/c/g/packages/spikes/json_client is an invalid path. Please supply a valid root.

So that the LLM knows better what it did wrong. check operator used on a null value is too vague.