utf8: Safeguard against use of old *and* new option

The new 'utf8foldernames' will not work together with the existing
'decodefoldernames' option (which will be documented in the next
commit). Therefore this commit will check for this condition and
abort the synchronization of a misconfigured account before doing
any changes.
Other accounts are not affected.

Signed-off-by: Urs Liska <git@ursliska.de>

offlineimap/accounts.py

@@ -274,6 +274,21 @@ class SyncableAccount(Account):
                 raise
             return
 
+        if self.utf_8_support and self.remoterepos.getdecodefoldernames():
+            e = OfflineImapError("Configuration mismatch in account " +
+                        "'%s'. "% self.getname() +
+                        "\nAccount setting 'utf8foldernames' and repository " +
+                        "setting 'decodefoldernames'\nmay not be used at the " +
+                        "same time. This account has not been synchronized.\n" +
+                        "Please check the configuration and documentation.",
+                    OfflineImapError.ERROR.REPO)
+            self.ui.error(e, exc_info()[2],
+                          msg="Configuration mismatch in account " +
+                        "'%s'. "% self.getname())
+            # Abort *this* account without doing any changes.
+            # Other accounts are not affected.
+            return
+
         # Loop account sync if needed (bail out after 3 failures).
         looping = 3
         while looping: