Fix crash when loading unsupported Tilt handlers

Tilt supports lots of different templating engines, each via a different
class.

When Tilt deprecates and removes a templating engine (as recently
happened with Maruku), Ruby will raise a `NameError` exception when we
call `Tilt.const_get` to access the (now missing) class by name.

This patch ignores those errors, allowing Nesta to try loading another
Markdown engine.

CHANGELOG.md

@@ -11,6 +11,15 @@
 
   (Suggested by Brad Weslake)
 
+* Handle errors raised when loading missing templating engine.
+
+  When a templating engine is removed from the Tilt library (as recently
+  happened with Maruku), Ruby was raising a `NameError` exception when Nesta
+  attempted to load it. Nesta now ignores those errors, and tries to load
+  another Markdown processor.
+
+  (Graham Ashton, reported by Curtis Cooley)
+
 * Fix whitespace in version.rb when generating new plugins. (Graham Ashton)
 
 ## 0.18.0 (19 November 2024)

lib/nesta/models/file_model.rb

@@ -1,6 +1,6 @@
 def register_template_handler(class_name, *extensions)
   Tilt.register Tilt.const_get(class_name), *extensions
-rescue LoadError
+rescue LoadError, NameError
   # Only one of the Markdown processors needs to be available, so we can
   # safely ignore these load errors.
 end