improve validation error messages on imported elements

[yangcao77]

Since the library do validation on flattened devfile, the current error message only contains the element name, does not contain the original source. It is unclear and hard to trace, library should do similar as DWO is doing: https://github.com/devfile/devworkspace-operator/blob/81b939ec513099fbb7f94959bdd5c6f37c47c62d/pkg/library/annotate/plugins.go#L22

An attribute can be added to the element before merge, validator (in devfile/api repo) can include this information in the error message

library.devfile.io/imported-by=<source-location>

This issue also expect change on devfile/api repo, as the validation is actually done in devfile/api validator.
validator will check for the specific attribute before sending back the error message. If the attribute does not exist, the error message will be as what it currently has.