Feature: error states

[mmastrac]

All errors are currently fatal, but we could be adding either error states (like Rust's Result) or try/catch keywords.

[mmastrac]

Possible forms for error states:

var f = file.open("foo.txt")?;
var s = file.read();
if let error(e) = s {
  // ...
} else if let ok(e) = s {
  // ...
}

Eventually this will allow us to use constructs like match, and the syntax will also work with optional types.