Change how error messages are shown

Instead of throwing the syntax errors with their source file location and needing to then catch them and call a `prettyErrorMessage` function in order to get the formatted error message, now syntax errors know how to pretty-print themselves (their `toString` method gets overridden).

An intermediate `catch` & re-`throw` is needed at the level of `CoffeeScript.compile` and friends. But the benefit of this approach is that now libraries that use the `CoffeeScript` object directly don't need to bother catching the possible compilation errors and calling a special function in order to get the nice error messages; they can just print the error itself (or let it bubble up) and the error will know how to pretty-print itself.
Override the SyntaxError's "stack" property instead of deleting it

This makes the "stack" property more useful when it's shown on other Node.js applications that compile CoffeeScript (e.g. testing libraries) and should fix #3023. A minimal example:

    $ node -e 'require("coffee-script").compile("class class")'

    /usr/lib/node_modules/coffee-script/lib/coffee-script/coffee-script.js:41
            throw err;
                  ^
    [stdin]:1:7: error: unexpected CLASS
    class class
          ^^^^^