WebSession: migrate to exception logging
- Migrates error logging to new exception-based technique. (addresses #580)
- Creates classes 'InvenioWebSessionError' for errors and 'InvenioWebSessionWarning' for warnings in 'websession_config.py'.
- Creates function 'tmpl_error()' to display errors.
- Removes function 'tmpl_warning' and rename function 'tmpl_warnings' with 'tmpl_warning'.
- Deletes CFG_WEBSESSION_ERROR_MESSAGES and CFG_WEBSESSION_WARNING_MESSAGES vars.
- Deletes errors var whenever it is needed.
- Updates the comments of functions.