Feature: Add rawError option

[rafoss]

Description

Introduces a new boolean option rawError to GuardProvider and GuardedRoute which makes them pass through errors thrown by GuardFunctions unaltered.

What this does

It alters the behaviour of error handling as seen here to use the raw thrown value instead of trying to access the message property on it and returning "Not found." as a fallback.

Use case

Custom errors such as extended Error objects which specify further details or entirely custom types thrown by GuardFunctions can now be accessed on the error prop of the Error page component.
A GuardFunction could for instance throw 403; to indicate HTTP status code 403 Forbidden to the Error page so it can conditionally render a forbidden page. This way it's not necessary to supply different error props per GuardedRoute you want to display different errors for.

Sidenote

In my opinion, a breaking change to supply the raw error by default would be nicer, but may cause more problems than it solves, so I've submitted this PR as a backwards compatible solution.

How to test

No test changes were made.

[joshpensky]

Hey @rafoss! Thank you so much for submitting this PR (and apologies for taking so long to review it)!

We agree with your sentiment of replacing the default error prop with the raw error object and are hoping to move forward with this idea as a breaking change! With #84, we're planning to land this change in 1.1.0 (after some non-breaking changes make it into 1.0.3)

Thank you again for your contributions! 🙌