Add useful Show instances for exception types

[snoyberg]

• Plugins.PluginException
• Exception types in Network.HTTP.Download (please unify into a single exception type, there are three right now)

[radix]

I don't know if this is the right ticket, but I had an idea:

• write documentation for each directive in stack.yaml
• make errors about each directive in stack.yaml link to specific sections in that documentation

[snoyberg]

@radix I think that comment was intended for a different issue.

[radix]

@snoyberg yeah, I actually made the same comment on a more appropriate issue: #269. It can safely be ignored here :)