feat(runtime): give OS errors .code attributes

[AaronO]

This adds .code attributes to errors returned by the op-layer, facilitating classifying OS errors and helping node-compat.

Similar to Node, these .code attributes are stringified names of unix ERRNOs, the mapping tables are generated by tools/codegen_error_codes.js and derived from libuv and rust's std internals

[bartlomieju]

We should discuss this change in design meeting before landing it. Type definitions are not updated, and I'm not sure we want to always assign these codes to errors.

[piscisaureus]

Non exhaustive first-pass. Might help with node-compat work

My suggestion would be:

• Do the the error code lookup in Rust
• Look at the system error code using std::io::Error::raw_os_error().
• On unix you want to map like this:

   let code: &'static str = match e.raw_os_error() {
     libc::ENOENT { "ENOENT" },
     libc::EISDIR { "EISDIR" },
     // etc...
   };

  The error codes that might appear in node are defined in libuv.
• On windows, use libuv's mapping table.

[AaronO]

@bartlomieju @piscisaureus PTAL

[piscisaureus]

LGTM, please consider comments.