[overlay] improve overlay mismatch error message by specifying left/right side context

some error message improvements to indicate left vs right nodes

```
Document on line demo.yml:10 [matched demo.yml:1]
Map item key 'clients' on line demo.yml:11 [matched demo.yml:2]
Map item key 'client1' on line demo.yml:12 [did not match]
Expected number of matched left map items to be 1, but was 0
```

vs

```
left=demo.yml right=overlay.yml
left=:1 right=:10 [matched] Document
left=:2 right=:11 [matched] Map item key 'clients'
left=? right=:12 [error] Map item key 'client1'
Expected number of matched left map items to be 1, but was 0
```

vs

```
left=demo.yml:1 right=demo.yml:10 [matched] Document
left=demo.yml:2 right=demo.yml:11 [matched] Map item key 'clients'
left=? right=demo.yml:12 [error] Map item key 'client1'
Expected number of matched left map items to be 1, but was 0
```

originated from https://github.com/k14s/ytt/issues/94

/cc @jtigger