Discriminate error types in login.

Discriminate error types in login.

ClientError and ConfigError should be handled differently. The former
suggests you did something wrong in account setup, and you might be
able to recover by doing it again properly (e.g. not misspelling the
server hostname), so we print the error in the TUI and let the user
try again. But the latter probably means you have to sort out your
local account setup (e.g. XDG dirs aren't working, or %APPDATA% isn't
set), and that's more like the kind of fatal error where you quit the
client and fix it externally.

So I've made a new LoginError enum to act as the union of those two
types and also indicate which you have. ConfigError is now passed
back to terminate the TUI completely.

This also means I'm no longer abusing TopLevelError for the union
error type returned by e.g. confirm_registration_fallible. That was
bad in its own way because it prefixed 'mastodonochrome: error:' to
the error description as if it were reporting on the terminal.