Provide more informative error messages

[bbbco]

I noticed that the error/failure messages were not as clear about what was failing:

Installing robbyrussell/oh-my-zsh... Done. Took 1s.
Antigen: Failed to load plugin.
Installing zsh-users/zsh-completions... Done. Took 1s.
Seems robbyrussell/oh-my-zsh is already installed!
Installing reegnz/jq-zsh-plugin... Done. Took 1s.
...

This PR allows errors/warnings to be a bit more informative:

Antigen: Seems plugin lib from robbyrussell/oh-my-zsh is already installed!
Antigen: Seems plugin git from robbyrussell/oh-my-zsh is already installed!
Antigen: Failed to load plugin zsh-nvm from robbyrussell/oh-my-zsh.
Antigen: Seems plugin zsh-users/zsh-completions is already installed!

[appinteractive]

Why is that not merged? It's really annoying to be spammed with errors that do not say anything 😕