Provide ErrorResult type

Summary:
ErrorResult<T> is a convenience alias to avoid to write types like
Result<User, Box<dyn Error>>, to allow any object implementing the
standard library Error trait to serve as the error part of a result.

Test Plan:

pub fn run(config: Config) -> ErrorResult<()> {
	something_dangerous()?;

	Ok(())
}

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1660