[bugfix] Use a default result for allusers() if next() leads to StopIteration

- use a default result for allusers() if next() leads to StopIteration
- catch the NoUsernameError exception in login.py script and only
  show the exception message instead of raising it
- fix error messages handling

Bug: T326063
Change-Id: Ia41bc9d80fcfd2e9e88542201236ceab87c6508c
---
M pywikibot/login.py
M pywikibot/scripts/login.py
2 files changed, 28 insertions(+), 9 deletions(-)

diff --git a/pywikibot/login.py b/pywikibot/login.py
index 1b1b661..e284b98 100644
--- a/pywikibot/login.py
+++ b/pywikibot/login.py
@@ -1,6 +1,6 @@
 """Library to log the bot in to a wiki account."""
 #
-# (C) Pywikibot team, 2003-2022
+# (C) Pywikibot team, 2003-2023
 #
 # Distributed under the terms of the MIT license.
 #
@@ -133,7 +133,7 @@
 
         try:
             data = self.site.allusers(start=main_username, total=1)
-            user = next(data)
+            user = next(data, {'name': None})
         except APIError as e:
             if e.code == 'readapidenied':
                 pywikibot.warning("Could not check user '{}' exists on {}"
@@ -294,15 +294,16 @@
             self.login_to_site()
         except APIError as e:
             error_code = e.code
-            pywikibot.error(f'Login failed ({error_code}).')
+
+            # TODO: investigate other unhandled API codes
             if error_code in self._api_error:
-                error_msg = 'Username "{}" {} on {}'.format(
+                error_msg = 'Username {!r} {} on {}'.format(
                     self.login_name, self._api_error[error_code], self.site)
                 if error_code in ('Failed', 'FAIL'):
-                    error_msg += f'\n.{e.info}'
+                    error_msg += f'.\n{e.info}'
                 raise NoUsernameError(error_msg)
 
-            # TODO: investigate other unhandled API codes (bug T75539)
+            pywikibot.error(f'Login failed ({error_code}).')
             if retry:
                 self.password = None
                 return self.login(retry=False)
diff --git a/pywikibot/scripts/login.py b/pywikibot/scripts/login.py
index f47bc20..cbdd9a2 100755
--- a/pywikibot/scripts/login.py
+++ b/pywikibot/scripts/login.py
@@ -47,7 +47,7 @@
    moved to :mod:`pywikibot.scripts` folder
 """
 #
-# (C) Pywikibot team, 2003-2022
+# (C) Pywikibot team, 2003-2023
 #
 # Distributed under the terms of the MIT license.
 #
@@ -58,7 +58,7 @@
 import pywikibot
 from pywikibot import config
 from pywikibot.backports import Tuple, nullcontext
-from pywikibot.exceptions import SiteDefinitionError
+from pywikibot.exceptions import NoUsernameError, SiteDefinitionError
 from pywikibot.login import OauthLoginManager
 
 
@@ -109,7 +109,10 @@
     if logout:
         site.logout()
     else:
-        site.login(autocreate=autocreate)
+        try:
+            site.login(autocreate=autocreate)
+        except NoUsernameError as e:
+            pywikibot.error(e)
 
     user = site.user()
     if user:

