Rebuild the docs around tested examples; shrink README.v2.md to a pitch (#2978)

docs_src/handling_errors/tutorial003.py

@@ -0,0 +1,14 @@
+from mcp.server import MCPServer
+from mcp.server.mcpserver.exceptions import ResourceNotFoundError
+
+mcp = MCPServer("Bookshop")
+
+CATALOG = {"Dune": "Frank Herbert", "Neuromancer": "William Gibson"}
+
+
+@mcp.resource("books://{title}")
+def book(title: str) -> str:
+    """The catalog entry for one book."""
+    if title not in CATALOG:
+        raise ResourceNotFoundError(f"No book titled {title!r} in the catalog.")
+    return f"{title} by {CATALOG[title]}"