Prevent traceback when using non-email in login

There's a traceback when the username used to log via SMTPAUTH
in is not an email address:

=== before ===
```
[...] ERROR in app: Exception on /internal/auth/email [GET]
Traceback (most recent call last):
  File "/usr/lib/python3.9/site-packages/sqlalchemy/engine/base.py", line 1179, in _execute_context
    context = constructor(dialect, self, conn, *args)
  File "/usr/lib/python3.9/site-packages/sqlalchemy/engine/default.py", line 719, in _init_compiled
    param.append(processors[key](compiled_params[key]))
  File "/usr/lib/python3.9/site-packages/sqlalchemy/sql/type_api.py", line 1201, in process
    return process_param(value, dialect)
  File "/app/mailu/models.py", line 60, in process_bind_param
    localpart, domain_name = value.lower().rsplit('@', 1)
ValueError: not enough values to unpack (expected 2, got 1)
[...]
[parameters: [{'%(140657157923216 param)s': 'foobar'}]]
```

=== after ===
```
[...] WARNING in nginx: Invalid user 'foobar': (builtins.ValueError) invalid email address (no "@")
```

core/admin/mailu/internal/nginx.py

@@ -5,6 +5,7 @@ import re
 import urllib
 import ipaddress
 import socket
+import sqlalchemy.exc
 import tenacity
 
 SUPPORTED_AUTH_METHODS = ["none", "plain"]
@@ -90,15 +91,19 @@ def handle_authentication(headers):
         except:
             app.logger.warn(f'Received undecodable user/password from nginx: {raw_user_email!r}/{raw_password!r}')
         else:
-            user = models.User.query.get(user_email)
-            ip = urllib.parse.unquote(headers["Client-Ip"])
-            if check_credentials(user, password, ip, protocol):
-                server, port = get_server(headers["Auth-Protocol"], True)
-                return {
-                    "Auth-Status": "OK",
-                    "Auth-Server": server,
-                    "Auth-Port": port
-                }
+            try:
+                user = models.User.query.get(user_email)
+            except sqlalchemy.exc.StatementError as exc:
+                exc = str(exc).split('\n', 1)[0]
+                app.logger.warn(f'Invalid user {user_email!r}: {exc}')
+            else:
+                if check_credentials(user, password, urllib.parse.unquote(headers["Client-Ip"]), protocol):
+                    server, port = get_server(headers["Auth-Protocol"], True)
+                    return {
+                        "Auth-Status": "OK",
+                        "Auth-Server": server,
+                        "Auth-Port": port
+                    }
         status, code = get_status(protocol, "authentication")
         return {
             "Auth-Status": status,

core/admin/mailu/models.py

@@ -57,6 +57,8 @@ class IdnaEmail(db.TypeDecorator):
 
     def process_bind_param(self, value, dialect):
         """ encode unicode domain part of email address to punycode """
+        if not '@' in value:
+            raise ValueError('invalid email address (no "@")')
         localpart, domain_name = value.lower().rsplit('@', 1)
         if '@' in localpart:
             raise ValueError('email local part must not contain "@"')