Pass API error messages through so programmer can see what went wrong

When I get a `HelloSign::Error::BadRequest` exception, I'd like to know why it was bad. The API provides it in `:error_msg`, so let's pass it through as the exception message.

```
HelloSign::Error::BadRequest: No data for custom field Interest Rate
```
Ta-da! :bowtie:

(Thanks a million for your masterful and well-spec'd library!)