Fixes issue #2754: Unparseable IMAP responses block email import from inbox

Some users experienced an error while fetching emails over IMAP (see
GitHub issue below for complete traceback). This error was caused by
certain unparseable character sequences in the responses Zammad received
from users' configured IMAP servers. The Zammad dev team was not able to
reproduce this issue when fetching identical emails from its own
servers.

Details remain unclear, but we have tracebacks from two distinct
occurrences of this issue, and the problematic strings followed the same
format:

  * "\"Jetzt"
  * "\"RE:"

This commit rescues the error in question, records details to the
application log, and allows IMAP fetch to proceed to the next message.

app/models/channel/driver/imap.rb

@@ -208,6 +208,7 @@ example
     count_max             = 5000
     too_large_messages    = []
     active_check_interval = 20
+    result                = 'ok'
     notice                = ''
     message_ids.each do |message_id|
       count += 1
@@ -222,6 +223,18 @@ example
       message_meta = nil
       timeout(FETCH_METADATA_TIMEOUT) do
         message_meta = @imap.fetch(message_id, ['RFC822.SIZE', 'ENVELOPE', 'FLAGS', 'INTERNALDATE', 'RFC822.HEADER'])[0]
+      rescue Net::IMAP::ResponseParseError => e
+        raise if !e.message.match?(/unknown token/)
+
+        result = 'error'
+        notice += <<~NOTICE
+          One of your incoming emails could not be imported (#{e.message}).
+          Please remove it from your inbox directly
+          to prevent Zammad from trying to import it again.
+        NOTICE
+        Rails.logger.error "Net::IMAP failed to parse message #{message_id}: #{e.message} (#{e.class})"
+        Rails.logger.error '(See https://github.com/zammad/zammad/issues/2754 for more details)'
+        next
       end
 
       # ignore verify messages
@@ -300,7 +313,7 @@ example
 
     Rails.logger.info 'done'
     {
-      result:  'ok',
+      result:  result,
       fetched: count_fetched,
       notice:  notice,
     }