Add more contextual information to errors from external libraries

[aturon]

A number of Viceroy's error variants are automatic conversions from external library errors -- things like http's errors for invalid headers or URIs, etc.

Most of those underlying errors don't provide any contextual information, such as which header or URI was provided that failed to be valid.

Rather than automatically converting these errors, we should provide our own error variants with contextual information that we can present to users in the error trace.