gomplate should output which template was being parsed when an error is encountered

[hairyhenderson]

The root cause of #238 was that an unexpected file (a Vi swap file) was present. The (unhelpful) error message was:

> gomplate --input-dir templates --output-dir out -d config=config.toml
Error: template: template:3: unexpected {{end}}

I think that naming the template after the current template file being processed is the best course of action.

Because of the way the code is organized right now this won't be terribly elegant, so I think fixing this may involve some refactoring.