improve ui on invalid types for endpoints

tests/ui/endpoint/invalid_placeholders_ty.rs

@@ -0,0 +1,19 @@
+#[macro_use]
+extern crate gotham_restful;
+use gotham_restful::gotham::hyper::Method;
+
+#[derive(Resource)]
+#[resource(endpoint)]
+struct FooResource;
+
+#[derive(Debug)]
+struct FooPlaceholders {
+	foo: String
+}
+
+#[endpoint(method = "Method::GET", uri = ":foo")]
+fn endpoint(_: FooPlaceholders) {
+	unimplemented!()
+}
+
+fn main() {}