Provide better error message when running example without features

Even better would be if "cargo run --example server" would
automatically enable those features, but Cargo can't do that right now.

Cargo.toml

@@ -15,6 +15,10 @@ name = "bench"
 harness = false
 path = "benches/bench.rs"
 
+[[example]]
+name = "server"
+required-features = ["iron-handlers", "expose-test-schema"]
+
 [features]
 default = ["rustc-serialize"]
 nightly = []

examples/server.rs

@@ -2,7 +2,7 @@ extern crate iron;
 extern crate mount;
 extern crate logger;
 extern crate rustc_serialize;
-#[macro_use] extern crate juniper;
+extern crate juniper;
 
 use std::env;