Misleading error if import returns no docType

[matthias-ronge]

If import returns a field docType, but the ruleset does not know any field docType, then the field is displayed with yellow marking and no error message appears. (So it is correct.) However, if import does not return a field docType, then an error message says that there is no field docType in the rule set.

Goal: Clarify error message that error is in import XSLT, not tin the ruleset.