Bug 12930: Web installer does not show login errors

Bug 12930: Web installer does not show login errors

Implemented invalid_username_or_password template
variable being handed to Auth.tt. Removed indentation
changes and modification to return value of checkpw

Test plan:
See comment #2

Alternative test plan:
- Clear session cookies or close / open your browser
- Go to  [MY SERVER]/cgi-bin/koha/installer/install.pl
- Verify that message appears if wrong username/pw provided
- Verify that you can log in with databas administrator account

Tested with alternative test plan, works as expected. Title and
description in commit message adapted.
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>