Better warning messages

[tlienart]

Adds significantly improved warning messages when

• there's a problem with a page var definition
• there's an issue with RSS generation
• there's an issue with running code
• there's an issue with using a page var

and a few other such things that were causing often unhelpful warnings.

Addresses #544

Note: can be globally turned off passing the keyword show_warnings=false to serve. These are not Julia warning and so will not be affected by logging levels (and vice versa: using show_warnings=false will only suppress the Franklin-warnings