Fix-dockerpull-regex

[tobybellwood]

In #462 we introduced expanded regex to support pulling of more images inside Pygmy, with all manner of fallbacks for partial image refs:

<>/<>/<>:<>

However, yours truly managed to miss a couple of cases in the regex, namely:

• tags with periods in (d'oh)
• image names with periods in (it's actually valid)
• organizations with periods in them (it may happen)

Reference: https://github.com/opencontainers/distribution-spec/blob/main/spec.md#pulling-manifests

This PR adds support for those edge cases into the regex, and also adds a helpful error message return for when pygmy pull fails - most likely for a missing image, or bad regex

Closes #536