mustache compat: Don't barf on unknown variables.

examples/error_not_found.txt

@@ -1 +1 @@
-ERROR: 'foo' not found in context
+

mustache.js

@@ -158,7 +158,8 @@ var Mustache = function() {
       if(context[name] !== undefined) {
         return context[name];
       }
-      throw({message: "'" + name + "' not found in context"});
+      // silently ignore unkown variables
+      return "";
     },
 
     // Utility methods

test/mustache_spec.rb

@@ -37,7 +37,7 @@ describe "mustache" do
         print('ERROR: ' + e.message);
       }
     JS
-    run_js(js).should == "ERROR: 'x' not found in context\n"
+    run_js(js).should == "\n"
   end
   
   non_partials.each do |testname|