Display errors returned by handler in CloudWatch

[Observer42]

Issue #98

Description of changes:

Change log level from debug to error for errors returned by handler. With this change, programs with default log level (log::Level::Info) will display error in CloudWatch.

(The log level is usually configured in fn main:)

fn main() -> Result<(), Box<dyn Error>> {
    simple_logger::init_with_level(log::Level::Info)?;
    lambda!(handler);
    Ok(())
}

By submitting this pull request

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I confirm that I've made a best effort attempt to update all relevant documentation.

[davidbarsky]

Thanks for these changes, I'm sorry about the delay! I'll get a release out later today.