Add PropertyUnspecifiedError exception

If we have situation, when we must specify at
least one of two conflicted resources, and there
is no specified resource, we get StackValidationFailed
with error msg looks like 'Either resource_1 or
resource_2 must be specified'. There are a few places,
where this situation appears. So it will be reasonable
to add special exception for these situations.

Change-Id: Ib107eff4fe8030fdbb40aa267b9e2b394ff7a186
Merge "Add PropertyUnspecifiedError exception"