ResponseError requires Debug + Display, but it seems unnecessary

Currently ResponseError is defined as:

```
// TODO: flesh out documentation
pub trait ResponseError: fmt::Debug + fmt::Display {
```

However I see no real reason why it requires Display, or even Debug.

The default implementation indeed requires Display:

```
let _ = write!(helpers::MutWriter(&mut buf), "{}", self);
```

However this means that we cannot implement ResponseError for any types that don't implement Display and Debug, which is cumbersome, because sometimes I just want to return a static string without relying on whether the type implements Debug or Display, and even if it does, I don't really intend to use it.

Not sure what the correct way would be to leave the default implementation as is, but also allow non-display, non-debug types to implement it.