Mark debug messages for externalization

We have decided to adopt a tiered approach to marking strings in Puppet with gettext's _() function. Step Two is to mark strings that are logged at debug level, since these will be useful for customers diving into issues by turning on debug output.

Helpful tool: https://github.com/Magisus/string-wrapper

Larissa Lane will use the result of this marking to get an idea of additional budget requirements for actually translating these strings.