[data/values] improve error message when value is specified as a flag and not data values file

slack thread: https://kubernetes.slack.com/archives/CH8KCCKA5/p1576624551037500

current error messsage:
```
$ cat example.yaml
#@ load("@ytt:data", "data")
---
boo: #@ data.values.foo

$ ytt -f example.yaml --data-value foo="asdf"
Error: Processing data values: Overlaying additional data values on top of data values from files (marked as @data/values): Document on line ?: Map item (key 'foo') on line ?: Expected number of matched nodes to be 1, but was 0
```