Issue with --env-only

[chevdor]

Using --env and --env-only fails:

tera --env --env-only --template templates/release.md
thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', src/wrapped_context.rs:122:59
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Current workaround:

echo {} | tera --env --template templates/release.md --stdin

[chevdor]

That seems to be also an issue with:

tera --template templates/changelog.md /tmp/test.json 

[chevdor]

The issue here (the error could be nicer) is that we passed an array instead of an object.

[chevdor]

That was no bug actually and the improvement in the error message should help spot the issue.