Add more standardized error definitions

It would be nice to have some predefined errors like:

var (
    ErrNoCertAvailable = errors.New("no certificate available")
    ErrSomethingWrong  = errors.New("...")
    // TBD.
)

So the client code could do:

if errors.Is(err, certmagic.ErrNoCertAvailable) { // Do something here }

So far we have some tricky logic that selects a certificate for the connection, which involves certmagic, static certificates, and some internal tools.
Having the ability to check for particular error would be really helpful.

What would you like to have changed?

https://github.com/caddyserver/certmagic/blob/d2a72863e63a7f37fc4bb39859614807d550b9df/handshake.go#L404

Why is this feature a useful, necessary, and/or important addition to this project?

It allows checking for the specific errors via generic methods like errors.Is

What alternatives are there, or what are you doing in the meantime to work around the lack of this feature?

if strings.Contains(err.Error(), "no certificate available") { ... } 

Start at handshake.go around line 404 and trace the certificate-selection errors returned through certmagic and related certificate sources. Identify the stable conditions that should have predefined errors, then verify that callers can distinguish them with errors.Is. Done means the selected errors are exposed as package definitions and the relevant returned errors support those checks.