deposit: field messages fix

• Fixes field info/warning/error/success messages, according to Bootstrap's newest documentation; also, adds a variable that prepares for future changes on the 'error' field name.

Signed-off-by: Pedro Gaudêncio <pedro.gaudencio@cern.ch>
Reviewed-by: Javier Martin Montull <javier.martin.montull@cern.ch>