common/rust: Remove failure_ext dependencies
common/rust: Remove failure_ext dependencies

Summary:
```
- use failure_ext::Error;
+ use anyhow::Error;
```

This diff applies to common/rust the style that I would like to follow across all of fbcode. Now that we are no longer tying the whole codebase to failure 0.1's Fail trait, there is no longer a reason to have all these targets depend on failure_ext just for basic types like Error and Result.

I'll make a similar diff to do the same in Mononoke and another for all the rest of the small projects depending on failure_ext. Once these all land, we will be able to strip down failure_ext to just the parts that are implemented directly by failure_ext (future and stream extension traits, slog integration) and factor those into individual more accurately named libraries which are used by far fewer targets than the current failure_ext.

Reviewed By: jsgf

Differential Revision: D18822481

fbshipit-source-id: e581e9c7387530a3bf7a1822f5b1b205d5e64875

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