WebSubmit: migrate to exception logging

• Migrates error logging to new exception-based technique. (addresses #580)
• Creates class 'InvenioWebSubmitWarning' for general warnings.
• Removes errors and warnings vars whenever it is needed.
• Deletes CFG_WEBSUBMITADMIN_WARNING_MESSAGES var.
• Changes the type of result returned (integer) whenever it is needed because of the result returned is a 'WRN_'.
• Updates the comments of functions.
• Adds calls to 'gettext_set_language' to translate the exceptions messages.