Improve missing template error message

[jdkaplan]

The CLI will error when trying to use a migration template file that doesn't exist. An example of the current error message:

$ squill new --template missing --name this
Error: failed to render template: Template 'missing/new.up.sql' not found

This message comes almost directly from Tera. A better error message would include Squill-specific information:

• The full (relative?) path of the missing template(s)
• A special message if the template subdirectory (or the whole templates_dir) doesn't exist
• A special message if the templates_dir is unconfigured/default

A sketch of something better:

Error: failed to render template: `.squill/templates/missing` not found

Error: failed to render template: `.squill/templates/missing/up.sql` not found

Error: failed to render template: `.squill/templates` (templates_dir) not found

Error: failed to render template: no templates_dir configured

Implementation notes

It may be easier to track which template files exist by tracking them in a map explicitly instead of using the one big tera::Tera instance.

Multi-template support was added in #168