incorrect compile error on inferred error set from async function

[andrewrk]

In creating a test case for #821 I ran into this:

const std = @import("std");

export fn entry() void {
    const p = nonFailing();
    resume p;
    cancel p;
}

fn nonFailing() promise {
    return (async<std.debug.global_allocator> failing()) catch unreachable;
}

async fn failing() !void {
    suspend;
    return error.Fail;
}

const builtin = @import("builtin");
pub fn panic(msg: []const u8, error_return_trace: ?&builtin.StackTrace) noreturn {
    while (true) {}
}

compile:

$ zig build-obj test.zig
/home/andy/dev/zig/build/test.zig:13:21: error: function with inferred error set must return at least one possible error
async fn failing() !void {
                    ^