Use error-chain to define Ekiden errors

[kostko]

We should use the error-chain crate to define our errors in ekiden-common instead of the manual error classes we have now as this seems to be the idiomatic way to do errors in Rust. I already used it to define errors for ekiden-tools in #160 and it seems really nice.

[nhynes]

just FYI, you'll need to use error-chain = { ..., default-features = false } so that it doesn't try to pull down backtrace and compile backtrace-sys. There's likely a way to get this to use the rust-sgx-sdk backtrace but I haven't looked into this yet.

[nhynes]

This is also not a good first issue ;) Turns out that PoisonError is unlinkable since it holds a MutexGuard<'_, ...>.

[kostko]

Oops :-)

[kostko]

We should likely use something like the failure crate, but maybe we should wait until 0.2 which will just extend the improved Error trait from the standard library (Rust RFC 2504).

Also see rust-lang-deprecated/failure#287.

[kostko]

This was done in #1531 (failure is now used everywhere).