Fixing a panic in load_data

[stanistan]

If there is no response from the server, load_data would panic with: response status.

This patch removes the expect in favor of an error message that we couldn't get a response from the server for a given url.

Before:

Building site...
-> Creating 7 pages (3 orphan), 7 sections, and processing 1 images
thread 'main' panicked at 'response status', components/templates/src/global_fns/load_data.rs:216:29
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

After:

Error: Failed to render page '/Users/stanistan/dev/stanistan.com/content/invoice/01.md'
Reason: Failed to render 'invoice.html'
Reason: Could not get response status for url: http://localhost:3000/invoice/01

---

Following up, I started thinking about it would be great if the function called in the templates would -- by default -- have messages that told you that they failed. I'm playing around with automatically wrapping all the registered Terra functions with messages/context using Error::chain so that the above response would look more like:

Error: Failed to render page '/Users/stanistan/dev/stanistan.com/content/invoice/01.md'
Reason: Failed to render 'invoice.html'
Reason: Failed calling tera function 'load_data'
Reason: Could not get response status for url: http://localhost:3000/invoice/01

Are you interested in a PR for this?

[Keats]

Thanks!