Improve template syntax error output

[officialasishkumar]

Summary

• Display Handlebars template syntax errors with their user-facing message instead of raw Debug output.
• Add a regression test for malformed templates so raw TemplateError structs are not printed.

Testing

• GIT_CONFIG_GLOBAL=/dev/null CARGO_NET_GIT_FETCH_WITH_CLI=true cargo test -p habitat_common template_errors_do_not_display_debug_output

Fixes #1677.

[chef-expeditor]

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[Copilot]

Pull request overview

This PR improves end-user error output for Handlebars template syntax failures in habitat_common by switching from Rust Debug formatting (which exposes internal struct details) to the template error’s Display message, aligning with the UX issue reported in #1677.

Changes:

• Update habitat_common::error::Error’s Display implementation to render TemplateError via to_string() rather than Debug output.
• Add a regression unit test ensuring template syntax errors do not include raw TemplateError { ... } debug formatting and still mention the template name.