Updates the wording on error handling to improve clarity.
Updates the wording on error handling to improve clarity.

It was not previously clear when a contributor should use `error_chain`
and when they should use `main()`. This patch adds a couple sentences to
the section on error handling to make that more explicit.

brson/rust-cookbook,budziq/rust-cookbook,brson/rust-cookbook,brson/rust-cookbook,budziq/rust-cookbook,budziq/rust-cookbook