Make more errors verbose

[killercup]

In #551 I added the ability to easily add details to std::io::Errors while not changing the type. This can make many error cases much more user friendly:

async-std/src/fs/file.rs

Lines 117 to 118 in b3d30de

	std::fs::File::open(&path)
	.context(|| format!("Could not open {}", path.display()))

I only added this in two places so far, though, File::open and File::create. Let's use this issue to track more places we should do this!

[gilescope]

Would love that std's errors could be just as helpful! Great work.

[killercup]

Thanks! That also reminds me: this issue can probably be closed by now. Feel free to enhance more errors of course!