Implement "fail EXPR" for convenience

[pcwalton]

log_err "Going down in flames..."; fail;" code is very common. It'd be nice to able to optionally supply an error message with fail statements.

[lkuper]

I've been trying to replace log_err "Going down in flames..."; fail; wherever it appears in the compiler with calls to cx.sess.err (or .unimpl or .bug or .span_err, whatever's appropriate), all of which have a fail. But fail that takes an optional error string would be good for user code.

[pcwalton]

Fixed in 42ac269, awesome!