[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
[FEATURE] Option in TS constant to prevent "parent decides" field display

By setting plugin.tx_fluidpages.siteRootInheritance = 0 (default value is 1, a value not set is considered 1 as well) the "parent decides" field for "page layout - this page" is hidden if the page being edited is a site root (is_siteroot = 1).
Merge pull request #49 from NamelessCoder/exceptiontemplates

[TASK] Further improve built-in FE exception template and BE handling
[TASK] Hide "parent decides" option for pages in pid zero.
[TASK] Tolerate and report errors in BackendLayout

This makes fluidpages exit silently and use NOTICE debug messages in the backend layout processor class when no template is selected, when configuration cannot be detected, when template contains errors and when template does not contain a grid.
[TASK] Special WARNING debug message if native view not configured
[TASK] Custom WARNING debug messages on path / file problems
Merge pull request #50 from NamelessCoder/selectorinherit

[FEATURE] Options and behavior to hide "parent decides" for root-type pages
[TASK] Report configured instead of resolved path when path does not exist
Merge pull request #51 from NamelessCoder/errortolerance

[TASK] Improve error tolerance and provide better debug messages