maint: Fix comment

* dietncl/xmlsugar.lua (match): Likewise.
maint: Add 'bad id-ref' error message

* dietncl/errmsg.lua (messages): Likewise.
maint: Handle <importBase> 'region' and 'baseId'

* dietncl/filter/import.lua (import_base): Add support to 'region' and
'baseid' parameters.
(resolve_importbase): Fix the return value if import_base() fails.
* tests/test-filter-import.lua: Add new checks.