UPDATE [15 Jul'15] : Salesforce support confirmed that this is indeed a bug and their Research & Development team is working on this case and the matter is under our observation. Here is the Known Issue record for this Bug : https://success.salesforce.com/issues_view?id=a1p300000008ZDq
When I try to query ValidationRule via REST (Tooling) API it's returning weird error:
UNKNOWN_EXCEPTION : An unexpected error occurred. Please include this ErrorId if you contact support: 1901783396-177948 (-6049864)
On debugging further I found that ValidationRule queried via REST (Tooling) API returns DELETED objects Validation Rules too (which in turn are deleted, obviously). And as it contains Deleted Validation Rule so the REST response returns error. Seems Salesforce Issue.
If I update my query (ie query Id ValidationName and TableEnumOrId) to following, no error is returned. Then I manually checked each validation rule and found out that one of VR (is deleted one) is of deleted object, hence the error.
Seems the error only comes if I include Metadata column as part of query else it doesn't throw error.
Salesforce should update VR REST API to return VR of NON DELETED objects. Is that the solution? or Am I missing something ?