Add more standardized error definitions #365

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?

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") { ... }