Handle errors gracefully

Right now, errors are not handled in a way... 

```shell
$ awsudo -u invalid echo bar
thread 'main' panicked at 'Profile not found', src/main.rs:34:27
note: Run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
```

it would be better to use something like:
```shell
$ awsudo -u invalid echo bar
Error: Profile not found
````