🔒 Verify SASL authentication has completed

[nevans]

The client is responsible for raising an error if the command completes successfully but "done?" returns false.

This is needed in order to correctly implement SCRAM-*:

• SCRAM-SHA-1(-PLUS) + SCRAM-SHA-256(-PLUS) + SCRAM-SHA-512(-PLUS) + SCRAM-SHA3-512(-PLUS) supports #54
• 🔒 Add SASL SCRAM-SHA-* mechanisms #172

Without this PR, if the server returns OK before sending its final message, then we can't satisfy the following requirement from the RFC:

The client then authenticates the server by computing the
ServerSignature and comparing it to the value sent by the server. If
the two are different, the client MUST consider the authentication
exchange to be unsuccessful, and it might have to drop the
connection.

Note that this PR adds Net::IMAP::SASL::Error (inheriting from StandardError, not from Net::IMAP::Error) and three subclasses: AuthenticationFailed, AuthenticationError, and AuthenticationCancelled.

Note that even EXTERNAL and ANONYMOUS send data (the authorization identity or the anonymous message/email), so even they are not done until process has been called at least once.