[TASK] Further improve built-in FE exception template

This change implements Partial templates named according to the exception code, these Partial templates can be filled out along the way to suggest solutions to the most common exceptions thrown by Flux and Fluidpages.
[TASK] Catch "file not found" Exception and send NOTICE message

This allows the "file not found" Exception to be handled gracefully in the backend - which may happen for sys folders (or other folders which are outside the page tree for whatever reason).

The previously fatal error thrown by Flux itself is now caught by FluidPages (when trying to read the backend layout from the template). Replacing the fatal error with a NOTICE allows it to be hidden from view when Flux debug mode is set to staging or production mode (0 or 1).
[TASK] Make debug flash message more generic

Since multiple types of Exceptions may be encountered it's necessary to make the message generic.
[TASK] Facilitate graceful failures on invalid template selection
[TASK] Use "NoSelection" fallback structure on empty selection
[TASK] Return NULL when failing getTemplateVariables
Merge pull request #49 from NamelessCoder/exceptiontemplates

[TASK] Further improve built-in FE exception template and BE handling