replace homegrown Error with failure crate

This reduces boilerplate, making it a bit easier for me to split the db stuff
out into its own crate.

src/main.rs

@@ -35,6 +35,7 @@ extern crate core;
 extern crate docopt;
 extern crate futures;
 extern crate futures_cpupool;
+#[macro_use] extern crate failure;
 extern crate fnv;
 extern crate http_serve;
 extern crate hyper;
@@ -67,7 +68,6 @@ mod coding;
 mod cmds;
 mod db;
 mod dir;
-mod error;
 mod h264;
 mod json;
 mod mp4;