ldapauth: fix crash on entering wrong password

LDAP_ is wrong prefix for unused passwords as it triggers exception
for wrong passwords:
ValueError: Unknown password hashing algorithm 'LDAP_Xnmn9ZRne376RMcSQK6YrUfd2rqmjaZP'.
Did you specify it in the PASSWORD_HASHERS setting?
'/usr/bin/python' => '/usr/bin/env python'

Unifying shebang lines, those were the only ones not using `env`.
`does_not_exists` => `does_not_exist`

Only used in this file, so hopefully not an API breakage.
--noinput for `pootle init`

Required a slight restructuring in program flow and a reworded message.
Moved filenames/paths to variables.