Improve handling of unknown protocols / missing plugins
Instead of failing to load the config, a fake prpl is created to load
the account, keep its settings, and refuse to log in with a helpful
error message.
Also added a new explain_unknown_protocol() function which returns text
which attempts to explain why a protocol is missing, handling several
typical cases, including the future removal of several dead libpurple
plugins.
That message is shown when logging in to a loaded account with a missing
protocol and when adding a new one with 'account add', with the
difference that the latter doesn't leave a placeholder fake account.
