Convert lapin-async's Error type to failure

[kureuil]

Uses the same hack as lapin-futures' ErrorKind type to prevent exhaustive matching against the different kinds of errors.

Incidentally, I also had to increase the type length limit to 16 millions characters to have the tests pass on Travis, the code was unable to compile otherwise which is pretty strange.

[Keruspe]

Maybe we should add some note wrt the type_length_limit in the README and somewhere in the doc?

Otherwise that’s great, thanks!

[kureuil]

I re-investigated the problem today, and removed 3 out of 5 occurences of type_length_limit. The 16 millions type limit is only needed in the examples/client.rs file, I suppose the nested futures are the real culprits for such a big value. I left a small comment on the remaining occurences hinting at the possible cause.

I don't think it's necessary to add a note to the README because this seems to be a relatively common problem when dealing with nested futures, and the compiler explicitely tells you to add this attribute to your crate when the problem arise, so the fix is easier than it might seem.

EDIT: It also doesn't occur on all platforms, I was unable to reproduce it on Windows, while I can consistently hit it when working on Linux. I don't know how it reacts on macOS, but it's probably more similar to Linux than Windows ?