fix config url checking

* In 0866b239, while fixing a clippy error, I accidentally inverted the
  error condition.
* While I'm at it, improve the diagnostics. Print which field we're
  talking about and the expected URL schemes.

server/Cargo.toml

@@ -36,6 +36,7 @@ h264-reader = "0.6.0"
 http = "0.2.3"
 http-serve = { version = "0.3.1", features = ["dir"] }
 hyper = { version = "0.14.2", features = ["http1", "server", "stream", "tcp"] }
+itertools = "0.10.0"
 lazy_static = "1.0"
 libc = "0.2"
 log = { version = "0.4" }