Login error handling message for invalid username in en lexicon

Username error message shows which user accounts are configured.
Attacker can guess the username. Message should be more generic so that
an attacker cannot guess a configured user account.

(cherry picked from commit 681ca406f16448f16bc166e03ce4759a5f0ba975)
Login error handling message for invalid username in other lexicons

non-english lexicons

(cherry picked from commit 9d6088383e72d6987eebf7c7fd183d607773fb42)

Conflicts:
	core/lexicon/es/login.inc.php
	core/lexicon/et/login.inc.php
	core/lexicon/fr/login.inc.php
	core/lexicon/it/login.inc.php
	core/lexicon/nl/login.inc.php
	core/lexicon/pt-br/login.inc.php
	core/lexicon/sv/login.inc.php
	core/lexicon/th/login.inc.php
Login error handling message for invalid username