RFC: rename UVError to IOError

[JeffBezanson]

fixes #7841

This is a bit tricky because the error sources we have use libuv-specific codes. So I tried this approach:

1. Rename UVError to IOError. It still has an integer code, but the code should be considered implementation-defined.
2. When libuv wrapper code throws an error, it uses the constructor _UVError(...). That function converts the error code to a message using the knowledge that it's a libuv code.

That way you always consume IOErrors, and they're self contained, but you can still use the code if you happen to know what it means.

[JeffBezanson]

I should have marked this for triage. @Keno @vtjnash any objections?

[StefanKarpinski]

Yes, just do it.

[samoconnor]

See #7841 (comment) -- Should IOError be abstract to allow future addition of fine-grained error types in a non-breaking way?