Consider remove error_chain dependency

[svartalf]

Related to rust-lang-deprecated/error-chain#240, tera::Error instances are !Sync.

For example, it is impossible to return tera::Error directly from the actix-web handler, and it is easier just to suppress it: https://github.com/actix/examples/blob/master/template_tera/src/main.rs#L28

Since tera::Error is quite simple, it might be worth to replace error_chain! call with a plain enum or use the failure crate.

[Keats]

That's a breaking change so for the next major version.

Related recent discussion: https://www.reddit.com/r/rust/comments/8lt8k6/do_i_really_need_failureerrorchain/

Failure seems nice but not really actively maintained so maybe going the plain enum would be better, depending on the timeline for the next major version

[Keats]

Looks like some of Failure is coming to Std so it would be better to wait before making a breaking change: https://www.reddit.com/r/rust/comments/92kozl/feedback_needed_upcoming_failure_012_release/e36pf5w/?context=0

[Keats]

So the way to go is probably plain std Error enum since using failure would probably be a dead end as things get moved to std. I am not sure how to provide the same UX as with error-chain though where errors are chained pretty seamlessly to give the whole context.

Once we have it working, we can also add more error types like the ones from #322