Switch to a newer error-handling mechanism than error_chain

[Dustin J. Mitchell]

Even with error_chain 0.12.0 (the latest):

warning: use of deprecated item 'std::error::Error::cause': replaced by Error::source, which can support downcasting
  --> src/error.rs:1:1
   |
1  | / error_chain! {
2  | |     errors {
3  | |         HeaderParseError {
4  | |             description("Unparseable Hawk header")
...  |
11 | |     }
12 | | }
   | |_^
   |
   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)