Add `errors` example demo file

[tcharding]

Add an errors example that creates and prints various error types from the crate. This is useful for two main purposes:

• Helps us catch mistakes by exercising error paths.
• Helps us check that error output is useful for debugging.
• Helps us check that output for "alloc" builds is good/useful (no-std)

Run with appropriate features to verify the output, e.g.,

      `cargo run --example errors --no-default-features --features=alloc`

This example is prone to go stale, is it useful? I could not think of any other way to verify that our error output was good (in both std and no-std environments).

[apoelstra]

Hmm, concept ACK 61b6300ad854067c55ef9ce27c3ea3749c74b5c5.

I think in practice this'd be pretty hard to keep in sync with the actual error types. Maybe it should be a collection of unit tests rather than examples?

[tcharding]

Its only about the "meaningfulness" of the error messages, I couldn't think of a way to test this except look at it every now and again. Once we are happy I don't think it will matter if some are missed, since devs will tend to copy the error style currently there.

[apoelstra]

Ok, makes sense.

After running the example all the error messages look great to me except that we should drop code= on the InvalidChar message.

[tcharding]

Sweet! Thanks for running them.

[apoelstra]

Do you want this to be merged?

[tcharding]

Maybe just leave it here and I'll have a go doing it in another repo, then see if it feels like its worth having. I'm not 100% sure it won't just sit and rot never to be touched again.