Add UI metadata options #28

Problem Statement

In addition to the error message, many error screens also include a contextual icon to reduce cognitive load and often a 'Retry' button. It would be good if Throwable including the built-in errors had the ability to provide:

1. A sensible icon based on the error type
2. A sensible default as to whether this error can be effectively retried
   • For example, most but not all network errors can be immediately retried, but validation errors can't be retried until the input changes.

Suggested Solution

Most likely some optional methods/variables added to the Throwable protocol, with values provided for all built-in errors.