Properly invalidate exceptions

The simple authenticator now properly invalidates its credentials. Also, the
invalidation functions have been given better names and documentation.

PFERD/authenticators/simple.py

@@ -1,6 +1,6 @@
 from typing import Optional, Tuple
 
-from ..authenticator import Authenticator, AuthSection
+from ..authenticator import Authenticator, AuthException, AuthSection
 from ..conductor import TerminalConductor
 from ..config import Config
 from ..utils import agetpass, ainput
@@ -42,7 +42,26 @@ class SimpleAuthenticator(Authenticator):
 
             if self.password is None:
                 self.password = await agetpass("Password: ")
-            else:
-                print("Password: *******")
 
         return self.username, self.password
+
+    def invalidate_credentials(self) -> None:
+        if self.username_fixed and self.password_fixed:
+            raise AuthException("Configured credentials are invalid")
+
+        if not self.username_fixed:
+            self.username = None
+        if not self.password_fixed:
+            self.password = None
+
+    def invalidate_username(self) -> None:
+        if self.username_fixed:
+            raise AuthException("Configured username is invalid")
+        else:
+            self.username = None
+
+    def invalidate_password(self) -> None:
+        if self.password_fixed:
+            raise AuthException("Configured password is invalid")
+        else:
+            self.password = None