Add error-chain throughout the codebase

[Michael-F-Bryan]

To help with #14 (better error messages), I've included error-chain as a dependency and updated almost all the places we return a Result to use the new error type instead of Box<Error> or std::io::Error.

I skipped the handlebars helpers though. Handlebars expects you to return a specific error type, so you can't use the error-chain error type there.

Now we just need to let people know to use the new error type when returning a Result, and that they should use chain_err() to return an error while also attaching a human-readable explanation of what went wrong.

[Michael-F-Bryan]

Travis just errored on the x86_64-unknown-linux-musl build with a rather interesting error message. Apparently while trying to compile backtrace-sys (a dependency of error-chain) the build script failed with:

configure: error: in `/home/travis/build/azerupi/mdBook/target/x86_64-unknown-linux-musl/debug/build/backtrace-sys-204dc57c91e9a514/out':
configure: error: C compiler cannot create executables

This may be an issue in the environment travis uses because it happened on stable, beta, and nightly for just that platform. For now I'll see if excluding the backtrace feature for that target platform works.

[Michael-F-Bryan]

Yay, travis is passing again :)

@azerupi and @budziq, I had to add a couple things to the x86_64-unknown-linux-musl build rules in travis.yml to get this to work. error-chain depends on backtrace-sys and on the default travis image you can't compile C libraries using musl. I had to add the following to travis.yml for all x86_64-unknown-linux-musl targets so backtrace-sys will build properly.

dist: trusty
addons:
  apt:
    packages: &musl_packages
      - musl
      - musl-dev
      - musl-tools

[azerupi]

Thanks! Looks good :)

[messense]

This caused mdbook test only print the lines below on failure with no detail

[*]: Testing file: "/Users/messense/Projects/PyO3/guide/src/./conversions.md"
An error occured:
A subprocess failed