Validate illegally-linked objects and nest errors inside IllegalLinkError

[dsleiter]

Coming from the conversation here: https://github.com/hdmf-dev/hdmf/issues/514

Following #515, when an illegally-linked dataset or group (a LinkBuilder is observed where a dataset or group spec has linkable=False) is encountered, that object will not be validated.

As an improvement, we would like that:

1. illegally-linked objects do get validated, but
2. any validation errors produced while validating the illegally-linked object should be nested inside the IllegalLinkError for that object

Additional context Note that as linkable=False is rarely used (as of the creation of this issue), this is probably not a very high-priority issue.

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