[v3] Streamline error types

[lestrrat]

Over the years I've had several reports about wanting to differentiate error types.

When I initially started developing this module, the state of error reporting in Go was still kind of meh, fmt.Errorf didn't exist, errors.Join didn't exist, etc. Opaque errors were recommended but I did not want to keep adding new error types or values ad infinitum, so I went the route of using (now deprecated) github.com/pkg/errors, which subsequently got replaced by fmt.Errorf

But now that I'm preparing to release v3, I believe the codebase is more or less stable and I can safely add error types/values as needed, and the required tooling now exist in Go itself.

General approach:

• Deprecate all IsXXXXError, use errors.Is
• Use specific error types (b/c I like it that way)
• Use utility function to wrap errors