This significantly changes the structure of potential output of
top-level errors. Before, errors that have a `source` were printed on a
single line, with the `source` omitted. Now, an error with a `source`
will be printed like:
```
Error: <`Display` impl of error>
Caused by:
<`Display` impl of `source` error>
```
If there are multiple `source` errors in a chain, then it will be of the
form:
```
Error: <`Display` impl of error>
Caused by:
0: <`Display` impl of first `source` error>
1: <`Display` impl of `source` of `source` error>
# etc.
```
Now, in practice this never happened before, since the only error
type that ever could return from `<binary>::run` was `std::io::Error`,
which has no `source`. However, `source`s can start being added if, say,
future work involved using `anyhow::Context::context` or custom errors
that implement `std::Error::source` are added to the codebase. I believe
this is a good choice going forward, but I definitely want to foster
discussion and acceptance for it in the open first!
Erich Gubler