[FEATURE] More informative error page with troubleshooting info
[TASK] Use new shortcut method to get stored variable from page template
[TASK] Make PageConfigurationProvider return extension key of carrier extension

This analyses the page record being rendered and returns the extension key related to the page template currently selected or inherited.
[TASK] Don't isolate detection of PageConfiguraionProvider to fluidpages scope
[TASK] Simplify custom controller handling in Page Controller
Merge pull request #32 from NamelessCoder/controllererrorhandling

[FEATURE] More informative error page with troubleshooting info