Ruby TemplateEvaluationContext handles missing ERB

This change updates the exception catching in the embeded ruby script
(template_evaluation_context.rb) so that when the supplied ERB template
argument is not a file on disk the exception handling code does not
itself error.

Prior to this change if the ruby code attempted to load a non-existent
ERB template the exception handling code would fail tyring ot access the
ERB instance's `#filename` method.

Author: aramprice

templatescompiler/erbrenderer/erb_renderer_test.go

@@ -134,6 +134,16 @@ property3: default_value3
 						Expect(err.Error()).To(ContainSubstring("RuntimeError: test error"))
 						Expect(err.Error()).To(ContainSubstring(rubyExceptionPrefix))
 					})
+
+					Context("with missing ERB template", func() {
+						It("returns an error with a known ruby exception", func() {
+							invalidErbPath := "invalid/template.erb"
+							err := erbRenderer.Render(invalidErbPath, renderedTemplatePath, context)
+							Expect(err).To(HaveOccurred())
+							Expect(err.Error()).To(MatchRegexp(fmt.Sprintf("No such file or directory .* %s", invalidErbPath)))
+							Expect(err.Error()).To(ContainSubstring(rubyExceptionPrefix))
+						})
+					})
 				})
 			})
 		})

templatescompiler/erbrenderer/template_evaluation_context.rb

@@ -181,7 +181,7 @@ def render(src_path, dst_path)
   rescue Exception => e
     name = "#{@context.name}/#{@context.index}"
 
-    line_i = e.backtrace.index { |l| l.include?(erb.filename) }
+    line_i = e.backtrace.index { |l| l.include?("#{erb&.filename}") }
     line_num = line_i ? e.backtrace[line_i].split(':')[1] : "unknown"
     location = "(line #{line_num}: #{e.inspect})"